Video report by ITV News Royal Editor Chris Ship
The announcement from Buckingham Palace that the Duke of Edinburgh is in hospital comes after months of speculation about his health.
Prince Philip has been living on the Sandringham Estate since his retirement and it was from there he was driven to London this morning.
He did not travel by ambulance. There was no ‘emergency’ situation.
But his doctors clearly decided on Thursday night that he needed what the Palace said was "observation and treatment".
The Duke walked into the King Edward VII hospital in central London on Friday morning.
Royal aides are not being drawn on the "pre-existing condition" for which he is being treated.
He has been treated in hospital before for a bladder infection.
And he was also in the same hospital for a hip replacement operation in 2018 – shortly before Harry and Meghan’s wedding.
How long will he be in for?
We don’t know – but it will be at least for a couple of days – which means the Duke may not be with the rest of the Royal Family on Christmas Day.
The Queen, however, did not see fit to change her travel plans on Friday.
As her husband was heading to hospital in London, she was heading from Buckingham Palace to Sandringham for the start of her Christmas stay there.
She traditionally remains at the Norfolk estate until early February, which is the anniversary of her father’s death.
King George VI died at Sandringham on February 6 in 1952.
Clearly, at age 98, everyone is concerned for the Duke of Edinburgh’s health.
At this stage, we simply do not know how serious his condition is.