Photo may show how Charles was infected
Prince Charles met the Queen just a day before he may have become infectious with coronavirus.
The 71-year-old tested positive for the bug on Monday night UK time after he began to display mild symptoms - but remains in "good health".
Medical experts believe the earliest Charles would have been contagious was March 13.
However, new research has found that symptoms could take up to 11 days to appear, meaning he could have potentially been infectious a day earlier, when he met the Queen 'briefly'.
This is very much a worst-case scenario, with the average incubation period of COVID-19 being five days.
A study by the Johns Hopkins Bloomberg School of Public Health in the US found that almost all (97.5 per cent) of those who develop symptoms appeared to do so within 11.5 days of infection.
Preliminary studies also show around 17.9 per cent of people with COVID-19 are asymptomatic, meaning they do not show any symptoms.
Sir Patrick Vallance, the UK's chief scientific adviser, said: "It looks quite likely that there is some degree of asymptomatic transmission.
"There's definitely quite a lot of transmission very early in the disease when there are very mild symptoms."
Research by Chinese government medical advisers has also revealed that the incubation period for coronavirus could be up to 24 days.
It is believed Prince Charles suffered mild symptoms over the weekend while at Highgrove House in Gloucester and flew privately to Scotland on Sunday night where he was tested on Monday.
Camilla, 72, tested negative for the virus, with the couple now self-isolating at their home in Balmoral, Scotland.
Clarence House said: "In accordance with government and medical advice, the Prince and the Duchess are now self-isolating at home in Scotland.
"The tests were carried out by the NHS in Aberdeenshire where they met the criteria required for testing.
"It is not possible to ascertain from whom the Prince caught the virus owing to the high number of engagements he carried out in his public role during recent weeks."
The royal met with Prince Albert of Monaco, who later tested positive for coronavirus, on March 10.
The Prince had been avoiding shaking hands with members of the public at a number of events recently, instead choosing to use the namaste gesture.
He last appeared in public with the Queen, along with princes William and Harry and other senior royals, at the Commonwealth Service at Westminster Abbey on March 9.
A spokesman for Buckingham Palace said: "Her Majesty the Queen remains in good health.
"The Queen last saw the Prince of Wales briefly after the investiture on the morning of March 12 and is following all the appropriate advice with regard to her welfare."
They would not comment on whether the Queen has been tested for coronavirus.
Prince William remains in Norfolk meaning the heirs to the throne are not in physical contact.
It comes as the UK death toll hit 422 yesterday in the biggest daily leap of fatalities.
This story originally appeared on The Sun and is republished here with permission
Originally published as Photo may show how Charles was infected