The Duke of Edinburgh was admitted to hospital overnight.
The 99-year-old taken as "a precautionary measure on the advice of His Royal Highness’s Doctor, after feeling unwell", Buckingham Palace confirmed in a statement.
He is to remain in hospital for a few days of observation and rest.
It’s understood the Duke travelled to hospital by car and it was not an emergency admission.
He walked into the building unaided and is said to be in "good spirits" according to reports from some British media.
It is not believed to be linked to coronavirus.
Philip has endured a number of ailments over the years, including being treated for a blocked coronary artery in December 2011 and a bladder infection in June 2012, forcing him to miss the Queen’s diamond jubilee celebrations.
In 2013, he had exploratory surgery on his abdomen and he began using hearing aids by 2014, aged 93.
In December 2019, he was treated for a "pre-existing condition" and was later discharged on Christmas Eve.
The Queen remains at Windsor.