The palace said the monarch is experiencing mild “cold-like symptoms” but expects to continue “light duties” at Windsor over the coming week.
“She will continue to receive medical attention and will follow all the appropriate guidelines,” the palace said in a statement.
The Queen had been in contact with her eldest son and heir, the Prince of Wales, who tested positive last week.
The Queen has tested positive for Covid just days after Charles and Camilla both caught the virus, but has still found time to send a message to Team GB's medal winners today.
The monarch, 95, is understood to be experiencing ‘mild cold-like symptoms', but is expected to continue with light duties at Windsor over the coming week.
She will continue to receive medical attention and will follow all appropriate guidelines.
Her Majesty's diagnosis comes after a number of people at Windsor Castle, where the monarch resides, tested positive for Covid-19.
It also comes just two weeks after the Queen reached her historic Platinum Jubilee, celebrating 70 years on the throne on February 6.
Buckingham Palace said in a statement today: ‘Buckingham Palace confirms that The Queen has today tested positive for Covid.
‘Her Majesty is experiencing mild cold-like symptoms but expects to continue light duties at Windsor over the coming week.
‘She will continue to receive medical attention and will follow all the appropriate guidelines.'
The diagnosis comes as the Queen today sent a message of congratulations to Team GB's women's curling team after they won a gold medal at the Winter Olympics in Beijing.
It is likely she will be working from her red boxes, sent to her every day and containing policy papers, Foreign Office telegrams, letters and other State papers from Government ministers and Commonwealth representatives that have to be read and, where necessary, approved and signed.
Prime Minister Boris Johnson today tweeted his well wishes to the monarch, saying: ‘I'm sure I speak for everyone in wishing Her Majesty The Queen a swift recovery from Covid and a rapid return to vibrant good health.'
The Queen is understood to be triple vaccinated but has been subject to health concerns since mid-October after cancelling a run of engagements and spending a night in hospital undergoing preliminary tests.
She missed the Remembrance Sunday event at the Cenotaph on November 14 last year due to a sprained back.
Prior to that, she missed a reception for business leaders at Windsor Castle on October 19 due to ill health, instead of spending a night at King Edward VII's Hospital. That hospital stay was her first in eight years when in 2013 she was treated at the private clinic for a bout of gastroenteritis.
The sovereign was also seen using a walking stick at a Westminster Abbey service in early October, the first time she had done so at a major event.
She is believed to have spent time with Charles on February 8, when he hosted an investiture at her Windsor Castle home before he tested positive a few days later.
The Duchess of Cornwall also tested positive for Covid, Clarence House confirmed on Monday, with a statement adding that the duchess was self-isolating.
Buckingham Palace repeatedly refused to disclose whether the Queen had herself contracted the virus, though it remains unclear exactly when she provided a positive test.
The Royal Household has its own royal physicians and the Queen's doctors will be on hand to take care of and monitor the head of state, with Professor Sir Huw Thomas, head of the Medical Household and Physician to the Queen, expected to be in charge.
During the pandemic, the Queen retreated to Windsor Castle for her safety was supported by around 24 staff in a so-called HMS Bubble.
The staff were split into two groups of 12 who worked away from their families on a ‘three weeks on, three weeks off' basis before the tight restrictions were relaxed as lockdown rules were loosened across the country.
Until recently she had been on doctors' orders to rest since, but the monarch carried out her first major public engagement for more than three months on February 5, the eve of her Jubilee, when she met charity workers at Sandringham House and cut a celebratory cake – using a walking stick to rest on.
And she admitted to feeling frail for the first time this week but managed a smile as she returned to work at Windsor Castle, despite scandals engulfing Princes Charles and Andrew that threaten to overshadow her Jubilee year.
Her Majesty again leaned on a walking stick as she told incoming Defence Services Secretary Major General Eldon Millar and his predecessor Rear Admiral James Macleod ‘I can't move' in the Oak Room at her Berkshire home on Wednesday.
The monarch stood rooted to the spot and leaned on her cane as she pointed to her left leg or foot – but mustered a smile when the two men approached her and shook her hand.
The Queen was understood to have been feeling slightly stiff, rather than having injured herself or being unwell.
Royal biographer Robert Jobson, author of William: The Making of a Modern Monarch, said the Royals are ‘concerned' about the Queen's positive test.
He told The Mirror: ‘There is no doubt The Prince of Wales and his family will be concerned by this development and won't be complacent.
‘Charles often spends longer at Windsor Castle when he visits these days so that he can enjoy quality time with his mother.
‘Kate, Duchess of Cambridge flies to Copenhagen this week for a series of solo engagements. If there was a serious cause for concern, I am sure that the overseas visit would have been postponed.
‘That said, every time there is a palace bulletin about the health of the monarch, it focuses the minds of her family and her loyal subjects during this Platinum Jubilee year.'
Royal biographer Angela Levin also told Sky News: ‘It is a very worrying time because you don't know what can happen and when.
‘You don't want to totally isolate her. She needs contact with people – she has done that ever since she came to the throne.
‘It would make her sad and think about negative things because that is what you do when you are on your own for a long time.'
She also said her Platinum Jubilee celebrations may face a rethink due to her health.
Royal commentator Alastair Bruce added: ‘It is a worry always when someone is much older, but she had a great deal of energy and is very fit.
‘This is a very real challenge to anyone at the age she is. The Queen is well looked after and has a huge interest in the work she does. She is a fighter.'
Royal writer Joe Little, of Majesty magazine, said the monarch will respond to her coronavirus diagnosis in a ‘matter-of-fact' and ‘stoic' manner.
He added: ‘I think the Queen is very stoic in every way. I would guess that she will be matter-of-fact about the diagnosis in a way perhaps that the people around her are less matter-of-fact.
‘In the coming days a very close eye will be kept on her and the indications are that, all being well, it's nothing more than a minor inconvenience.'
Government ministers have also sent their well wishes to the monarch on Twitter.
Education Secretary Nadhim Zahawi said: ‘Wishing Her Majesty the a swift recovery. God Save The Queen.'
Oliver Dowden added: ‘Wishing Her Majesty the Queen a swift recovery.'
Health Secretary Sajid Javid tweeted: ‘Wishing Her Majesty The Queen a quick recovery.'
While Chancellor of the Exchequer Rishi Sunak said: ‘Wishing Her Majesty a speedy recovery.'
And London Mayor Sadiq Khan tweeted: ‘The commitment Her Majesty the Queen has shown to our country continues to be unwavering. Wishing her a swift and safe recovery from Covid-19.'
Elsewhere Priti Patel said: ‘Wishing Her Majesty a quick recovery. God save the Queen. ‘
While Labour leader Keir Starmer wrote: ‘On behalf of myself and the whole of @UKLabour, wishing Her Majesty The Queen good health and a speedy recovery. Get well soon, Ma'am.'
Today royal expert Robert Jobson told GB News: ‘The Queen is 96 next month and Covid of course whether you've been vaccinated three times, as the Majesty, it still has an impact particularly on older people. I'm sure they're monitoring her very carefully.
‘I would think the idea of light duty as well. I think she should probably be focused on her health rather than light duties and it is a concern no doubt about that. My feeling is that hopefully that she'll just focus on health and not work.'
While the Queen has received her booster jab, which affords a high degree of protection against severe infection, she could be prescribed one or more of a number of anti-viral medications designed to protect the most vulnerable.
Professor Paul Hunter, an infectious disease expert at the University of East Anglia, said someone in their 90s would be at increased risk of severe disease compared to younger people, even if they have been triple vaccinated.
Nearly all severe Covid infections begin with mild symptoms, he explained.
Prof Hunter said: ‘With somebody in their mid-90s, even if they're triple vaccinated you are concerned that they could gradually deteriorate over coming days and so you would need to keep a very careful eye on them.
‘You would, I think, almost certainly be considering giving anti-viral drugs, of which there is a number around at the moment.'
He added: ‘If you do get them early enough it does reduce the risk of severe disease developing so I would imagine any doctor for a patient in their 90s would be considering giving these antivirals out.'
The positive Covid test comes during a stressful period for the Queen as Prince Andrew finalised the settlement of a bombshell sex assault case against him earlier this week, while the Met has also launched a probe into cash for honours scandal involving Charles's charitable foundation.
The Duke of York has agreed to an undisclosed out-of-court settlement with Virginia Giuffre after she sued him claiming she was trafficked by paedophile Jeffrey Epstein, Andrew's friend, to have sex with the royal when she was 17 and a minor under US law.
But it came amid fresh fury over claims the Queen contributed towards the estimated £12million deal agreed with his accuser.
The monarch is expected to foot part of the bill for her son's settlement in a bid to draw a line under it before her much-anticipated Platinum Jubilee celebrations this summer.
But there is anger at how the Queen has effectively been forced to bail out the ‘disgraced' Duke of York, 61, whose modest pension from his time in the Royal Navy is now his only visible income – amid calls for the public to be told who is financing the deal.
Andrew marked the quietest royal birthday in modern history yesterday as he looks to lay low after settling his sex abuse case.
The muted celebration of his 62nd birthday came just hours after the death of Jeffrey Epstein's ‘pimp', who was accused of trafficking hundreds of girls to the paedophile financier.
French modelling agent Jean-Luc Brunel, 76, who allegedly procured more than a thousand women and girls for Epstein to sleep with, was found hanged in his prison cell in an alleged suicide on Saturday morning.
And Charles could also be questioned by the Met Police as part of its criminal ‘cash for honours' investigation into his aide and charity.
The Royal, 73, nor anyone in his household has been spoken to as part of the force's assessment on whether to probe how a Saudi billionaire donor got a knighthood and UK citizenship after donations to the Prince's Foundation.
Instead, officers have liaised with staff at the charity, who have provided documents for the scoping exercise.
But Charles and his former right-hand aide Michael Fawcett, 59, have been both explicitly named in the two letters to the Met that sparked the investigation.
Fawcett stepped down in October amid claims he promised to help secure a CBE and British citizenship for Mahfouz Marei Mubarak bin Mahfouz – a donor to the Prince's Foundation.
It was alleged that he paid thousands of pounds to ‘fixers' with links to the prince who said they could secure him an honour in return for donations.
Clarence House has denied Charles knew anything about the allegations being investigated.
And Prince Harry the monarch has also been dealing with revelations that Prince Harry tried to keep details of his legal battle to reinstate his police protection secret from the public.
High Court documents show he sought a far-reaching confidentiality order on documents and witness statements surrounding his case against the Government.
But the Home Office argued for transparency, saying ‘there must be a sufficiently good reason, in the wider public interest, to justify the departure from open justice that such an order involves'.
The Queen's positive test comes as Boris Johnson today urged people to be ‘more confident and get back to work' as he heralded this coming Thursday as Covid Freedom Day.
The PM gave an upbeat assessment ahead of unveiling his ‘Living with Covid Plan' tomorrow, insisting vaccines and new treatments can be relied upon to keep the public safe.
All curbs – including legal self-isolation – are set to end in England within days, and Mr Johnson made clear that the taxpayer cannot keep shelling out £2billion a month on mass testing.
In a compromise between the Treasury and Department of Health, he will lay out a timetable for axing free tests – but they are still likely to be available for more vulnerable and older age groups.
As cases continue to tumble – down 25 per cent week-on-week – Mr Johnson insisted he did not want people to ‘throw caution to the winds' but he wanted to remove ‘compulsion' and let individuals take responsibility.
A timeline of events leading up to the Queen's Covid-19 diagnosis
Prince Charles tests positive for Covid-19 for a second time and begins self-isolating.
The royal met with the Queen just two days before he tested positive for the virus and had spent time with her at Windsor Castle.
Buckingham Palace refuses to confirm whether the 95-year-old monarch has tested positive or negative for Covid, fuelling fears for her health.
The Duchess of Cornwall tests positive for Covid-19.
Royal sources say Camilla, 74, has been triply vaccinated, adding that Clarence House will continue to follow government guidelines and review her engagements.
Buckingham Palace officials said they would “not be providing a running commentary” on the Queen's health.
A Clarence House spokesman said: ‘The Duchess of Cornwall has tested positive for Covid-19 and is self-isolating. We continue to follow government guidelines.'
The diagnosis comes as doctors continue to monitor the Queen's health after Charles was diagnosed with Covid-19 less than 48 hours after seeing his mother.
Buckingham Palace announces the Queen has tested positive for Covid-19.
The monarch, 95, is understood to be experiencing “mild cold-like symptoms”, but is expected to continue with light duties at Windsor over the coming week.
A number of cases have been diagnosed in the Windsor Castle team, according to reports.