Prince Philip has undergone a "successful procedure" for a pre-existing heart condition.
The 99-year-old Duke of Edinburgh was transferred from King Edward VII's hospital to St. Bartholomew's in London on Monday and he will stay there for "a number of days" in order to recover from the undisclosed treatment.
An official statement said: "The Duke of Edinburgh yesterday underwent a successful procedure for a pre-existing heart condition at St Bartholomew's Hospital.
"His Royal Highness will remain in hospital for treatment, rest and recuperation for a number of days."
Philip was admitted to hospital last month after feeling unwell and has spent 16 nights there so far in what is his longest ever stay while being treated for an infection.
A statement from Buckingham Palace announced earlier this week: "The Duke of Edinburgh was today transferred from London's King Edward VII’s hospital to St Bartholomew’s hospital where doctors will continue to treat him for an infection, as well as undertake testing and observation for a pre-existing heart condition.
“The Duke remains comfortable and is responding to treatment but is expected to remain in hospital until at least the end of the week."
Meanwhile, Prince Edward recently said his father is "looking forward" to getting out of hospital soon but is "a bit" frustrated about staying.
He said: "As far as I'm aware, well I did speak to them the other day, so he's a lot better thank you very much indeed, and he's looking forward to getting out, which is the most positive thing ... Just a bit [frustrated].
"I think that gets to all of us, and then you can only watch the clock so many times and the walls are only so interesting. We’ve had some brilliant and lovely messages from all sorts of people and we really appreciate that and so does he, I’ve been passing them on. It’s fantastic, thank you."