Prince Charles was with the Queen just a day before he may have been infectious with Covid-19.
The 71-year-old future king tested positive for the virus this week after showing mild symptoms, but is "in good health", according to The Sun.
Keeping up appearances despite the outbreak, Charles met with Prince Albert of Monaco on March 10 and sat opposite the Prince during a dinner. Prince Albert later tested positive for the virus.
Charles had been avoiding hand-shaking during the dinner, instead opting for a namaste gesture.
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A statement from 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."
Experts say the earliest he could have been contagious was March 13, but new research shows that symptoms can take up to 11 days to appear.
This means he could potentially have been infectious the day before he briefly met with the Queen.
The average incubation period of Covid-19 is five days. Preliminary studies show that around 17 per cent of people who contract the disease are asymptomatic.
It's believed Charles had mild symptoms over the weekend while at Highgrove House in Gloucester and then flew to Scotland, where he was tested on the Monday.
Camilla tested negative for the virus, and the couple are now self-isolating at their Balmoral home.
A Buckingham Palace spokesperson said the Queen remained 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."
The Palace did not comment on whether the Queen had been tested for coronavirus.