The Duke of Edinburgh has spent the night in hospital after being admitted as a "precautionary measure".
Prince Philip, 98, travelled from the Queen's Sandringham Estate, in Norfolk, to London's King Edward VII Hospital on Friday morning.
Buckingham Palace said the admission was for "observation and treatment in relation to a pre-existing condition".
The BBC's Simon Jones said the Royal Family will be hoping the duke is able to return to Sandringham for Christmas.
The palace said the duke went to hospital on the advice of his doctor. But it refused to confirm or deny reports the duke was flown to London by helicopter and then driven by car for the last part of the journey.
The duke walked into hospital and is expected to remain there for a few days.
As he was travelling to London, the Queen was on her way to Sandringham for the start of her Christmas break.
She caught the 10:42 GMT Great Northern service from London's King's Cross and was later pictured stepping off the train at King's Lynn railway station.
At the scene
Simon Jones, BBC News, outside King Edward VII Hospital
While Prince Philip remains in hospital, the surrounding streets are full of TV satellite trucks from around the world, waiting to broadcast the latest news on his condition.
Buckingham Palace hasn't released any further statements overnight - though that's to be expected. The Duke is known as a deeply private person who doesn't like a fuss.
The palace won't want to give a running commentary on any treatment he might be receiving.
For as long as it's unclear though why he's been admitted to hospital, speculation will continue - and one key question is whether he'll be well enough to join the Queen at Sandringham for Christmas.
Although there's concern about the duke's condition, in palace circles there's not a sense of alarm. I think we see that by the fact that the Queen has decided to continue with her Christmas traditions rather than remaining here in London.
Last Christmas, Prince Philip missed the royals' traditional Christmas Day trip to church.
Since retiring from official solo royal duties in 2017, the duke has appeared in public alongside the Queen and other members of the Royal Family at events and church services.
He was treated for a blocked coronary artery in 2011. The following year, he suffered a bladder infection and was forced to miss the Queen's Diamond Jubilee concert.
He was also taken to hospital for an abdomen operation in 2013 and, in 2014, underwent surgery on his right hand.
Last year he had a hip replacement at the same central London hospital that he is now attending.