Geoffrey Howe served as chancellor in Mrs Thatcher's first Cabinet, becoming foreign secretary in 1983 before being demoted in 1989 to leader of the Commons.
He became increasingly estranged from the prime minister because of his more enthusiastic approach towards Europe.
Labour's Denis Healey once said debating with him was like being savaged by a dead sheep. But Howe proved to be a lethal enemy within his own party, sparking the train of events that saw Thatcher forced to quit in 1990.
As Lord Howe, he remains an active pro-Euro campaigner.