Sophie, Countess of Wessex, is reported to be self-isolating at home after being in contact with someone who later tested positive for Covid-19.
The 55-year-old wife of Prince Edward has not reported any symptoms of the coronavirus, according to The Mail.
She is said to have met with an unnamed person who has since been diagnosed with the virus.
She has not been in contact with any members of the wider Royal Family since the meeting.
Sophie took part in the London Marathon on Sunday where 45,000 people across the country ran on their own or in small groups - rather than in a single gathering.
The Countess ran with a Mencap Learning Disability Running team for the charity.
A palace spokeswoman said: “Earlier this week the Countess of Wessex came into contact with someone who has subsequently tested positive for Covid-19.
“Her Royal Highness is not experiencing any symptoms, but is following all relevant Government guidelines and is self-isolating at home.”
