LONDON: Prince Philip, Queen Elizabeth's husband who helped modernise the monarchy and steer the British royal family through repeated crises during seven decades of service, died on Friday at Windsor Castle. He was 99.

The Duke of Edinburgh, as he was officially known, had been by his wife's side throughout her 69-year reign, the longest in British history. During that time he earned a reputation for a tough, no-nonsense attitude and a propensity for occasional gaffes.

"It is with deep sorrow that Her Majesty The Queen announces the death of her beloved husband, His Royal Highness The Prince Philip, Duke of Edinburgh," the palace said in a statement.

"His Royal Highness passed away peacefully this morning at Windsor Castle. Further announcements will be made in due course. The Royal Family join with people around the world in mourning his loss."

Flags at Buckingham Palace and at government buildings across Britain were lowered to half-mast and within an hour of the announcement the public began to lay flowers outside Windsor Castle and Buckingham Palace. The royal family's website was also shut down, replaced by a photo of Philip and the announcement of his death.

A Greek prince, Philip married Elizabeth in 1947. He went on to play a key role helping the monarchy to adapt to a changing world in the post-World War Two period, and behind the walls of Buckingham Palace was the one key figure the queen could trust and turn to, knowing he could tell her exactly what he thought. "He has, quite simply, been my strength and stay all these years," Elizabeth, 94, said in a rare personal tribute to Philip in a speech marking their 50th wedding anniversary in 1997.