Last Tommy dies at 111.
Mr Patch, known as the Last Tommy, passed away peacefully just before 9am at Fletcher House nursing home in Wells, Somerset.
He had become Britain's oldest man when another veteran of the war, Henry Allingham, died a week ago on July 18, aged 113.
The only remaining British-born veteran of World War I is now Claude Choules, 108, who served in the Royal Navy and now lives in Perth, Australia.
Mr Patch, a machine-gunner, served in the trenches as a private from June to September 1917 and fought in the battle of Passchendaele in which more than 70,000 British troops died.
He was born on June 17 1898 and grew up in Combe Down, near Bath.
The Queen said: "I was saddened to hear of the death this morning of Harry Patch, the last British survivor of the First World War.
"We will never forget the bravery and enormous sacrifice of his generation, which will continue to serve as an example to us all."
Prince Charles said nothing could give him greater pride than to pay tribute to Mr Patch.
He said: "The Great War is a chapter in our history we must never forget; so many sacrifices were made, so many young lives lost."
The Prime Minister added: "I know that the whole nation will unite today to honour the memory, and to take pride in the generation that fought the Great War."