Catherine, Duchess of Cambridge, will self-isolate after being in contact with a positive COVID-19 case, Kensington Palace said on Monday.
She was meant to join her husband Prince William at events celebrating the 73rd birthday of Britain’s National Health Service.
Contacts of people who test positive for COVID-19 must quarantine for 10 days in the United Kingdom.
Kate had her first dose of a coronavirus vaccine back in May and does not have any symptoms of the virus.
On Friday, she watched the Wimbledon Tennis Championships from the royal box.
COVID infections have been rising in the UK due to the spread of the more transmissible Delta variant.
Infections this past week increased by 66% compared to the previous week, with 24,248 new cases reported on July 4. Hospitalisations, however, are not rising as quickly.
The UK has high vaccination rates with 86% of the adult population having received a first dose of a COVID-19 vaccine and 63.8% of the population fully vaccinated.