The Duke of Kent is stepping down from his role as Colonel of the Scots Guards, Buckingham Palace has said.
Edward, who is Queen Elizabeth II's cousin, held the title for longer than anyone else, having assumed it in 1974.
He officially stepped down on Sunday after attending his final Scots Guards Black Sunday in Westminster and taking the salute for the last time.
The duke, 88, will be replaced as the head of the regiment by the Duke of Edinburgh, 60.
He described it as "a true honour, which will forever fill me with great joy".
"Through those years, I have seen the work of the Scots Guards during peacetime and war and witnessed their bravery, selfless courage and devotion to duty," he said.
"To my fellow Scots Guardsmen, I am immensely proud to have served you all. I am delighted that His Royal Highness The Duke of Edinburgh will continue to champion all that you do and work towards preserving your great legacy."
The Scots Guards is one of the five prestigious foot guards regiments of the army's Household Division.
They and the four others - the Irish ,Welsh, Grenadier, and Coldstream Guards, mostly take part in state events in support of the monarch, but are also deployed around the world.
Since the Scots Guards was established in 1642 its members have taken part in the Battle of Waterloo in 1815, First and Second World Wars, Falklands conflict, and those in Afghanistan and Iraq.
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The Duke of Kent had a 21-year military career, starting as a second lieutenant with the Royal Scots Greys before being promoted to the highest possible rank - field marshal.
The Duke of Edinburgh said: "Quite apart from his depth of knowledge and keen understanding of the regiment and all those who serve, past and present, the Duke of Kent has been a tireless and passionate advocate.
"It is a distinct honour to be asked and entrusted by His Majesty to serve as the next colonel.
"However, I accept with a degree of trepidation as I will undoubtedly be measured against the formidable record and reputation of my predecessor. I can only promise to do my best."