Prince Philip, Queen Elizabeth II's husband, has died aged 99, Buckingham Palace has announced.
Buckingham Palace said: "It is with deep sorrow that Her Majesty The Queen has announced the death of her beloved husband, His Royal Highness The Prince Philip, Duke of Edinburgh.
"His Royal Highness passed away peacefully this morning at Windsor Castle."
Boris Johnson said he "inspired the lives of countless young people".
Speaking at Downing Street, the prime minister added: "He helped to steer the Royal Family and the monarchy so that it remains an institution indisputably vital to the balance and happiness of our national life."
Mr Johnson said he received the news of the duke's death "with great sadness".
"Prince Philip earned the affection of generations here in the United Kingdom, across the Commonwealth, and around the world," he said.
Paying tribute to the duke's role as the longest serving consort in history, Mr Johnson also remembered Prince Philip as one of the last surviving people to have fought in the Second World War.
"From that conflict he took an ethic of service that he applied throughout the unprecedented changes of the post-War era," Mr Johnson said.
"Like the expert carriage driver that he was, he helped to steer the Royal Family and the monarchy so that it remains an institution indisputably vital to the balance and happiness of our national life."
'Deep sense of commitment'
Scotland's First Minister Nicola Sturgeon said she was "saddened" by the death of the duke.
She tweeted: "I send my personal and deepest condolences - and those of scotgov and the people of Scotland - to Her Majesty The Queen and her family."
Justin Welby, the Archbishop of Canterbury, said: "He consistently put the interests of others ahead of his own and, in so doing, provided an outstanding example of Christian service."
The archbishop added: "As we recover and rebuild after the terrible trial of the coronavirus pandemic, we will need fortitude and a deep sense of commitment to serving others."
The prince married Princess Elizabeth in 1947, five years before she became Queen, and was the longest-serving royal consort in British history.
In March, the Duke of Edinburgh left hospital after a month-long stay for treatment.
He underwent a procedure for a pre-existing heart condition at another London hospital - St Bartholomew's.
Prince Philip and the Queen had four children, eight grandchildren and 10 great-grandchildren.
Their first son, the Prince of Wales, Prince Charles, was born in 1948, followed by his sister, the Princess Royal, Princess Anne, in 1950, the Duke of York, Prince Andrew, in 1960 and the Earl of Wessex, Prince Edward, in 1964.
Prince Philip was born on the Greek island of Corfu on 10 June 1921.
His father was Prince Andrew of Greece and Denmark, a younger son of King George I of the Hellenes.
His mother, Princess Alice, was a daughter of Lord Louis Mountbatten and a great-granddaughter of Queen Victoria.
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