LONDON, April 9 (NBCNews)- Prince Philip, Queen Elizabeth II's husband and the longest-serving consort of any British monarch, has died at age 99.

Philip spent 65 years supporting the queen, retiring from his public role in 2017 and staying largely out of the view since. In his active years, he helped set a new course for the monarchy under a young queen, championing Britain itself, as well as environmental causes, science and technology.

Philip’s relationship with the young Princess Elizabeth began as a story of young love.

“We behave as though we had belonged to each other for years,” Elizabeth wrote in a letter to her parents shortly after they married.

Over the years, the queen acknowledged Philip’s deep influence on her, calling him her “strength and stay” in a speech on their 50th wedding anniversary in 1997.