Buckingham Palace said the monarch had accepted the foreign secretary's advice to delay the event, which was scheduled for Wednesday.

The Queen had to cancel planned virtual audiences twice this week following her COVID diagnosis, but it's understood that this postponement is because of the conflict in Ukraine.

The Queen would have met hundreds of members of the Diplomatic Corps at the event.

In a statement, Buckingham Palace said: "The Queen has accepted the foreign secretary's advice that the diplomatic reception at Windsor on March 2 should be postponed."

Following the announcement of her COVID diagnosis, the palace had said the 95-year-old monarch had been experiencing "mild cold-like symptoms".

While her virtual appointments were postponed, she did speak to Prime Minister Boris Johnson by phone for their weekly audience on Wednesday.

There was concern over the Queen's recent coronavirus diagnosis because of her advanced age and a recent health scare that left her in hospital late last year.

It comes after Prince Charles contracted COVID for the second time less than two weeks ago, with the Duchess of Cornwall testing positive days later.

Sky News understands a number of cases have been diagnosed within the Windsor Castle team.