The Duke of Edinburgh has an infection and is expected to remain in hospital for several more days, Buckingham Palace said on Tuesday.
Prince Philip, 99, was admitted to the King Edward VII hospital in London on February 16 on the advice of his doctor.
Buckingham Palace said the Duke would remain under observation while he is treated for an infection.
“The Duke of Edinburgh remains at King Edward VII’s Hospital where he is receiving medical attention for an infection,” the palace said, providing no further details of the illness.
“He is comfortable and responding to treatment but is not expected to leave hospital for several days.”
Both royals have received their first dose of the coronavirus vaccine and the Duke's illness is not related to Covid-19.
The Queen has remained at Windsor Castle, her home to the west of London, where the couple have stayed for most of the pandemic.
Prince Edward, the Duke’s youngest son, said the royal family was keeping their “fingers crossed” for a full recovery. He said his father was feeling “a lot better” a week after he was admitted to hospital.
"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,” he told Sky News on Tuesday.
"So we keep our fingers crossed."
Royal reporters said the latest statement from Buckingham Palace “suggested the infection is more serious than first thought”.
“Last week the statements were about ‘observation and rest’ and being in hospital because his doctor was acting only ‘an abundance of caution’,” ITV royal editor Chris Ship tweeted.
Prince William, son of heir Prince Charles, said on Monday his grandfather was "OK, they're keeping an eye on him".
Philip's illness comes at a tricky time for the queen and the monarchy after it was announced last week that William's younger brother Prince Harry and his American wife Meghan would be severing all their official royal links.
Harry, 36, and Meghan, 39, stepped down from duties in March last year, and that split was formalised on Friday, with the couple losing their treasured royal patronages.
Prince Philip has been the Queen's confidante throughout her 69-year reign and is the patriarch of the family.
However, the prince has taken more of a back seat since he retired from public life in 2017, with many of his official roles being taken on by other members of the family.