The Duke of Edinburgh was admitted to hospital after feeling unwell, Buckingham Palace said on Wednesday.
Prince Philip, 99, the husband of Queen Elizabeth II, was admitted to the King Edward VII Hospital in London on Tuesday evening.
"The Duke's admission is a precautionary measure, on the advice of His Royal Highness's doctor, after feeling unwell," a spokesman said.
"The Duke is expected to remain in hospital for a few days of observation and rest."
The prince, who celebrates his 100th birthday on 10 June, had felt unwell for a short period and walked from a car into the hospital unaided.
He does not have a Covid-19-related illness.
Last month, the palace announced that Prince Philip and the queen had both received their first Covid-19 vaccination at Windsor Castle.
A No 10 spokesperson said Prime Minister Boris Johnson sends his "best wishes" to the duke as he "undergoes a few days of rest in hospital".
He was last admitted to hospital in December 2019, spending four nights in the King Edward VII Hospital for what the palace said was planned treatment of a pre-existing condition.
In 2011, he was rushed to hospital by helicopter after suffering chest pains and treated for a blocked coronary artery. In 2017, he spent two nights in the King Edward VII hospital and was in hospital for 10 days in 2018 for a hip replacement.
He was forced to give up driving at the age of 97, after crashing into a car while driving a Land Rover near Sandringham estate in England, in January 2019. The Duke needed help to get out of the vehicle but was not injured. A woman in the other vehicle suffered a broken wrist.
He stepped down from official engagements in August 2017 and rarely appears in public.
During England’s current coronavirus lockdown, he is staying at Windsor Castle, west of London, with the Queen.
The royal couple received their Covid-19 inoculations last month.
