Former Conservative chancellor Geoffrey Howe has died at the age of 88 after suffering a suspected heart attack, his family has announced.
Lord Howe, Margaret Thatcher's longest serving cabinet minister and chancellor from 1979 to 1983, died on Friday.
His resignation speech in 1990 is widely seen as a key factor in Baroness Thatcher's downfall as prime minister.
Prime Minister David Cameron led tributes, saying: "The Conservative family has lost one of its greats."
The former MP for the East Surrey, Reigate and Bebington constituencies, played a "vital" role in "turning the fortunes of our country around" as chancellor, Mr Cameron added.
Lord Howe's family said he died at his home in Warwickshire after attending a jazz concert with his wife Elspeth.
During his time as an MP, Lord Howe also served as Mrs Thatcher's foreign secretary, deputy prime minister and leader of the House of Commons.
But BBC political correspondent Robin Brant said his resignation as deputy prime minister - shortly after Mrs Thatcher declared that the UK would never join a single currency project - had been a "devastating blow" to her premiership.
Lord Howe of Aberavon
- Born in Port Talbot, south Wales, on 20 December 1926
- Left Winchester College in 1944 and joined the Army, later going to Cambridge University to read law
- He was MP for Bebington from 1964-66, Reigate from 1970-74, and East Surrey from 1974-92
- Became chancellor of the exchequer under Margaret Thatcher in 1979
- Later served as foreign secretary, leader of the House, and deputy prime minister
- He resigned from cabinet in November 1990
- Left the Commons in 1992 and was made a life peer later that year
In a famous speech to the Commons, he used a cricket metaphor to describe Mrs Thatcher's attitude to British negotiations in Europe.
"It is rather like sending your opening batsmen to the crease, only to find... that their bats have been broken before the game by the team captain," he said.
Mrs Thatcher resigned shortly after, later saying he had become a "source of division and a focus of resentment".
'Modest with a steely conviction'
BBC political correspondent Robin Brant
A wolf in sheep's clothing - Geoffrey Howe was mild mannered and modest with a steely conviction. In the end he was unwilling to let Margaret Thatcher ride roughshod over him.
Denis Healey once famously said that being attacked by him was like being "savaged by a dead sheep". Yet Lord Howe proved devastating when he rose to make his resignation speech in the Commons after he quit the cabinet in protest at the then prime minister's approach to Europe and diplomacy.
His cricket analogy revealed the deep frustration many felt about what they had come to regard as an autocratic leader (who was already on the wane). With that the "dead sheep" inflicted the first blow.
He will be regarded as the man who delivered the radical changes that transformed the UK in the 1980s, by both those who regard that as a good thing and those who don't.
Mr Cameron described Lord Howe - who retired from the Lords in May - as "the quiet hero of the first Thatcher government".
"Geoffrey Howe was a kind, gentle and deeply thoughtful man - but at the same time he had huge courage and resolve," the prime minister said.
Chancellor Osborne tweeted: "I will miss Geoffrey Howe. He was a great source of advice to me; a quietly-spoken radical, whose bitterly contested budgets rescued Britain."
Sir John Major, who succeeded Lady Thatcher as prime minister, said Lord Howe "was a man whose political convictions, turned into law, transformed our nation".
He added: "Geoffrey has left a benevolent mark on our national life, and I hope in death will receive the credit so often denied him in life."
'Unbending and unyielding'
Other former Conservative chancellors were among those paying tribute. Lord Lamont praised him as a "Tory with a social conscience, who wanted opportunity for all".
Ken Clarke said: "He had very strong views about how best to serve the national interest - that's what he saw himself as doing, and having thought through his views with a very good, clear intellect he then stuck to them and succeeded in delivering them."
Former Deputy Prime Minister Lord Heseltine said he had been a "political rock".
"He carried huge personal strain for the success of that government," he added.
"They were very controversial times, controversial policies, but Geoffrey was unbending and unyielding but he was a very kind, courteous and gentle man."
Welsh Secretary Stephen Crabb said Lord Howe - who was born in Port Talbot - had been a "towering" political figure who always had a great affection for Wales.
Labour leader Jeremy Corbyn tweeted: "I got to know Geoffrey Howe when he was Foreign Secretary and valued his knowledge and experience. Condolences to Elspeth and family."
And Labour's shadow chancellor John McDonnell said: "Lord Howe devoted his life to the service of his country and did so with distinction.
"He was not afraid to stand up for what he believed in and famously demonstrated this in his historic confrontation with Mrs Thatcher."
Liberal Democrat leader Tim Farron described Lord Howe as a "kind, decent and honourable man".
A statement from Lord Howe's family said a private family funeral, followed by a memorial service, would take place in due course.