He was Margaret Thatcher’s mild-mannered, loyal and longest-serving Cabinet minister – a former Chancellor of the Exchequer, Foreign Secretary and Deputy Prime Minister.

But there had been tensions between Geoffrey Howe and the Prime Minister since 1982, when Thatcher refused to appoint him to her Falklands war cabinet. And when he threatened to resign in 1989, over the issue of Britain’s entry to the European Exchange Rate Mechanism, Howe was removed from the Foreign Office and made Lord President of the Council – effectively a demotion. He was later frozen out of Thatcher’s inner circle following the resignation of Nigel Lawson.

The final straw in Howe’s relations with Thatcher came in late 1990, when the eurosceptic Prime Minister famously said “No, no, no!” to Jacques Delors’ push for closer political and economic union in Europe (a stance backed by The Sun in its famous front-page headline ‘Up Yours, Delors’). Howe resigned on 1 November, penning a letter which criticised Thatcher’s handling of British relations in Europe. “I am deeply anxious that the mood you have struck – most notably in Rome last weekend and in the House of Commons this Tuesday – will make it more difficult for Britain to hold and retain a position of influence in this vital debate,” he wrote.

And the normally placid Howe was not going to go quietly, either. His resignation speech – delivered 12 days later to a packed House of Commons – took everyone by surprise, being uncharacteristically dramatic for a man whose attacks Denis Healey had likened to “being savaged by a dead sheep”.

Delivering his speech after Prime Minister’s Questions, Howe told the House that Margaret Thatcher’s “perceived attitude towards Europe is running increasingly serious risks for the future of our nation” and that he had tendered his resignation “with the utmost sadness and dismay.”

[Read more: March 16, 1976 - Prime Minister Harold Wilson announces resignation]

“I have done what I believe to be right, for my party and my country,” he concluded. “The time has come for others to consider their own response to the tragic conflict of loyalties, with which I myself have wrestled for perhaps too long.”

Kenneth Baker said some years later that Howe “knew what he was doing. He was, in fact, preparing the assassination of Margaret Thatcher.” Interviewed in 2013, however, Howe insisted: “I didn’t feel that I was going to bring her [Thatcher] down or anything like that, it wasn’t designed to achieve that. I felt I couldn’t go on wearing two hats.”

Either way, as Total Politics later described it: “By the time Howe sat down, after his clarion call to others to consider their own positions, everyone there felt they had just witnessed a political earthquake, and Tories knew that the PM's position was undermined, possibly fatally.”

Sure enough, a few days after Howe’s speech, Michael Heseltine launched his leadership challenge against Margaret Thatcher. She failed to win the first round of the subsequent leadership election outright – and resigned as Prime Minister and Conservative Party leader on November 22, 1990.