The Duke of Cambridge has said the Duke of Edinburgh is “OK” after his 99-year-old grandfather spent a sixth night in hospital.

William was asked about Philip’s health during a visit to a Covid-19 vaccination centre in Norfolk and lightened the mood by giving a playful wink.

At the end of his tour of the inoculation hub, set up in the King’s Lynn Corn Exchange, the duke was quizzed about Philip and replied: “Yes, he’s OK, they’re keeping an eye on him,” and gave a wink.

There will be concerns about the duke, who is now into his seventh day at the private King Edward VII’s Hospital in central London.

He was admitted on Tuesday evening as a precautionary measure after feeling unwell and walked into the building unaided.

Philip was initially due to spend a few days under medical care for “observation and rest” for the undisclosed reason, which is not coronavirus-related.

But on Friday sources told the PA news agency the duke was likely to remain in hospital into this week.

He received a visit from the Prince of Wales on Saturday – a rare occurrence as the duke is known for his “no fuss” attitude.

Sources have told the PA news agency that the duke’s doctor is “acting with an abundance of caution”.

Members of the monarchy have continued with their duties since Philip was admitted after telling his doctor he felt unwell.

The morning after he was admitted, the Prince of Wales and Duchess of Cornwall – who have received their Covid jabs – visited the Queen Elizabeth Hospital in Birmingham to learn about vaccination trials.

On Thursday, the Queen carried out her first official in-person event of the year and made one of her most senior royal aides a Knight Commander of the Royal Victorian Order during a Windsor Castle ceremony.

William, who contracted coronavirus last year, chatted to staff, volunteers and those receiving Covid jabs when he visited the vaccination centre in Norfolk.

He asked vaccinator Debbie Rudd, 50, whether the patients were nervous, adding: “I’d be at the front of the queue if I could, just to prove that it’s OK, but I have to wait my turn.”

Philip has been spending the latest lockdown with the Queen at Windsor Castle and in January they both received Covid vaccinations.

The duke last had a spell in hospital in December 2019, where he was treated for a “pre-existing condition” and discharged on Christmas Eve after four nights.