Kate Middleton is having to self-isolate after coming into contact with someone who later tested positive for coronavirus, Kensington Palace says.
Kate was due to spend the day with William celebrating the 73rd anniversary of Britain's National Health Service at two major events but will now have a 10-day period at home.
They had planned to attend a Service of Thanksgiving at St Paul's Cathedral in honour of the nation's health service, and in the afternoon host a Big Tea at Buckingham Palace for NHS staff, in their roles as Joint Patrons of NHS Charities Together.
"Last week The Duchess of Cambridge came into contact with someone who has subsequently tested positive for COVID-19," Kensington Palace said in a statement on Monday.
"Her Royal Highness is not experiencing any symptoms, but is following all relevant government guidelines and is self-isolating at home."
Kate's last public event was a visit to Wimbledon on Friday when she toured the SW19 sporting venue meeting staff in the museum, Centre Court kitchen and sitting with former tennis star Tim Henman to watch Jamie Murray play in the doubles.
Kate and Prince William were also pictured last week alongside their son, Prince George and stars David Beckham and Ed Sheeran at the Euros 2021.
The duchess is patron of the All England Lawn Tennis Club and a keen tennis fan but is now likely to miss the men's and women's final at the weekend.
Both Kate and William are vaccinated against COVID-19, with the Duchess of Cambridge receiving the jab in May.
“Yesterday I received my first dose of the Covid-19 vaccine at London’s Science Museum," a message on the Kensington Palace Twitter account read at the time.
“I’m hugely grateful to everyone who is playing a part in the rollout - thank you for everything you are doing.”
William previously contracted COVID and was able to carry on with telephone and online engagements while he was treated by royal doctors but was reportedly hit "pretty hard" by the virus and at one stage struggled to breathe.
The Prince of Wales also caught the virus during the same period but had milder symptoms although he did lose his sense of smell and taste for a period.
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