Prince Philip has been taken from the Sandringham estate to a London hospital for observation and treatment in relation to a pre-existing condition overnight.
In a statement, the palace said it was "on the advice of His Royal Highness' Doctor".
The duke, 98, retired from public life in August 2017 after spending decades supporting the Queen and attending events for his own charities and organisations.
Since retiring from official solo duties, he has appeared in public alongside the Queen and other members of the Royal Family at events and church services, but the duke has not been seen out in public since attending Lady Gabriella Windsor's wedding in May.
In a statement, the palace said they were playing things safe with this morning's actions.
"The Duke of Edinburgh travelled from Norfolk this morning to the King Edward VII Hospital in London for observation and treatment in relation to a pre-existing condition.
"The admission is a precautionary measure, on the advice of His Royal Highness' Doctor."
The duke is expected to be in hospital for a few days, the BBC reports. He was not taken by ambulance and it was a planned admission.
It comes as the Queen arrived in the royal Sandringham estate in Norfolk after leaving London yesterday.
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