Prince Philip, Queen Elizabeth's 98-year-old husband, was admitted to a London hospital on Friday for what Buckingham Palace described as a "precautionary measure." In a statement, the palace said Philip was admitted for observation and treatment in relation to a pre-existing condition on the advice of his doctor.

Philip had been at the royal family's Sandringham estate in eastern England, where the queen and her family usually spend Christmas. The palace said Philip didn't travel by ambulance and his admission wasn't an emergency. He is expected to be in the hospital for a few days.

The 93-year-old queen traveled to Sandringham from London on Friday morning after attending the State Opening of Parliament in London on Thursday. She did not change her schedule after her husband's hospital admission, which was planned in advance.

Get Breaking News Delivered to Your Inbox

The private King Edward VII hospital in central London has been treating members of the royal family for decades.

Philip has not been seen in public since attending Lady Gabriella Windsor's wedding in May, according to BBC News.

Prince Philip, Duke of Edinburgh attends the wedding of Princess Eugenie of York to Jack Brooksbank at St. George's Chapel on October 12, 2018, in Windsor, England. Alastair Grant / Getty Images

Philip, also known as the Duke of Edinburgh, married Elizabeth in 1947 and is the longest-serving royal consort in British history. He and the queen have four children, eight grandchildren and eight great-grandchildren.

A former naval officer and keen polo player, Philip enjoyed robust health well into old age but has had a number of health issues in recent years. He stopped attending public engagements in 2017.

In 2011, he was rushed to a hospital by helicopter from Sandringham after suffering chest pains and treated for a blocked coronary artery.

In 2017, he spent two nights in the King Edward VII hospital and was hospitalized for 10 days in 2018 for a hip replacement.

Philip was forced to give up driving earlier this year, at the age of 97, after smashing into a car while driving a Land Rover near Sandringham in January. Philip needed help to get out of the vehicle but wasn't injured. One woman in the car suffered a broken wrist.

He still drives a horse and carriage on private land from time to time.