It means Anne, who is isolating with Sir Tim, will not be able to join the 95-year-old monarch at Windsor Castle on Christmas Day, a senior royal source said.

       The Princess Royal, 71, and her husband, 66, had been set to spend the day at Windsor after the Queen cancelled plans to celebrate with a larger group of family members at Sandringham - the royals' traditional Christmas destination - due to concerns over Omicron.

       It is understood the couple will now remain at their Gatcombe Park estate in Gloucestershire, where they are isolating.

       Sir Tim, a retired Naval officer, contacted people on Monday to warn them he had tested positive.

       The Queen is still likely to have some family members over for lunch at Windsor Castle on Christmas Day.

       They are thought to include the Earl and Countess of Wessex and their children, Lady Louise Windsor and James, Viscount Severn. Prince Andrew is also expected to be there.

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       But other family members, including Prince Charles and Camilla and the Duke and Duchess of Cambridge, are still waiting to confirm their plans for where they will spend the day.

       The Queen is expected to attend a Christmas Day service in her private chapel at Windsor Castle.

       The pandemic has not only ruined the Royal Family's usual Christmas plans at Sandringham. Earlier this week, the Queen also cancelled her annual pre-Christmas lunch for up to 40 family members amid mounting concern over the spread of the Covid variant.