Prince Philip, the 99-year-old husband of Britain's Queen Elizabeth II has been hospitalised as "a precautionary measure", Buckingham Palace announced on Wednesday.
The Duke of Edinburgh was admitted to the private Kind Edward VII Hospital in London on Tuesday evening after feeling unwell. He is expected to remain there for a few days of "observation and rest", the Palace also said.
No detail was given although his illness is not believed to be related to COVID-19. Both the Queen and her husband received a first dose of the vaccine against the deadly virus in early January.
The royal couple has been staying in Windsor Castle in recent weeks as the country is in its third national lockdown, imposed following the emergence of a more transmissible variant.
Philip retired from public duties in 2017.
The Prime Minister's office said Boris Johnson "sends his best wishes to the Duke of Edinburgh as he undergoes a few days of rest in hospital".
Philip, who will celebrate his 100th birthday on June 10, was last hospitalised in December 2019. The Palace said at the time that his four-day stay at Kind Edward VII Hospital was to treat a pre-existing condition.
Prior to that, he underwent a hip replacement in 2018 and was treated for a blocked coronary artery in 2011.
Philip, a former naval officer, married the then-Princess Elizabeth in 1947 and is the longest-serving royal consort in British history. He and the queen have four children, eight grandchildren and nine great-grandchildren.