Britain's Prince Philip, the husband of Queen Elizabeth II, was admitted overnight to a London hospital as a "precautionary measure", Buckingham Palace said Wednesday.

The 99-year-old was taken to the private King Edward VII Hospital on Tuesday evening, the palace said, adding that it was "a precautionary measure, on the advice of His Royal Highness's Doctor" after the prince felt unwell.

The Duke of Edinburgh "is expected to remain in hospital for a few days of observation and rest," the statement said.

The prince and the Queen, 94, have been maintaining social distancing rules at Windsor Castle, because their age puts them at risk from Covid.

The couple received their first vaccination doses in January, the palace said in a rare public comment on the private health matters of the royal couple.

Prince Philip retired from public duties in 2017 at the age of 96 following a two-night hospitalisation for an infection. He then had a hip operation in 2018.

In January 2019, he emerged unscathed after his vehicle was involved in a traffic accident that injured two people near the monarch's Sandringham estate in eastern England.

Philip spent four nights at King Edward Hospital in December 2019, where he was treated for what was described as a "pre-existing condition" and though he was discharged on Christmas Eve that year, he has not been seen in public since.

The latest hospitalisation renewed concerns about Philip's health ahead of his 100th birthday in June this year.