Prince Philip, the 98-year-old husband of Queen Elizabeth II, is being treated at a hospital, according to Buckingham Palace.
The Duke of Edinburgh was taken to King Edward VII Hospital in London for "observation and treatment in relation to a pre-existing condition," the palace said in a statement.
"The admission is a precautionary measure, on the advice of His Royal Highness' Doctor," the statement read.
Philip walked into the hospital on his own for what was described as a planned admission to the hospital, according to ABC News royal contributor Victoria Murphy.
He traveled from Norfolk, England, where the queen has her country estate Sandringham, to London on Friday for the medical care.
On the same day, Queen Elizabeth traveled from London to Norfolk to begin her Christmas holiday. She will remain in Norfolk until Feb. 6, the anniversary of the death of her father, King George VI.
The royal family plans to spend Christmas at Sandringham, per tradition.
Prince Philip, who wed then-Princess Elizabeth in 1947, retired from official royal duties in 2017.
He had at that time completed 22,220 solo engagements since 1952, given 5,496 speeches in his travels to more than 76 countries, authored 14 books, served as patron to 785 organizations and made 637 solo overseas visits, according to Buckingham Palace.
One year after his retirement, in 2018, Philip underwent hip replacement surgery at the age of 96.
The Duke of Edinburgh was involved in a car accident at the start of this year. In January, Philip was driving a Land Rover that was involved in a collision with a Kia near Sandringham Estate.
Philip was uninjured in the accident, Buckingham Palace said at the time.
This is a developing story. Please check back for updates.