Former Conservative Cabinet minister Lord Mayhew, who served as attorney general and Northern Ireland secretary during critical moments for both posts, has died at the age of 86, his family said.

His family confirmed he died peacefully at his home in Kent on Saturday, June 25.

Conservative politician Patrick Mayhew was MP for Tunbridge Wells from 1974 until 1997, when he stood down at the general election.

Lord Mayhew was NI secretary and attorney general from 1992 to 1997, making him the longest-serving secretary of state for Northern Ireland. He was a key figure in the December 1993, Downing Street Declaration, formulated by then prime minister John Major and then Irish taoiseach Albert Reynolds, which led to the IRA ceasefire the following September.

And his selectively leaked letter – as solicitor-general in January 1986 – setting out the background to the bitter Westland Helicopters dispute, led, through no fault of his own, to the resignations of both the then defence secretary, Michael Heseltine, and the trade and industry secretary, Leon Brittan.

It even, for a few dramatic hours, imperilled the prime minister, Margaret Thatcher, herself. But she survived.

Lord Mayhew would have regarded himself on the “liberal” wing of the Conservative Party, but he was not the kind of politician who could easily be labelled.

Party intrigue did not attract or concern him. He was a man who, with the discipline that goes with being a Queen’s Counsel, concentrated on the job in hand to the exclusion of all else.

Patrick (Paddy) Barnabas Burke Mayhew was born on September 11 1929, and attended Tonbridge School and Balliol College, Oxford, where he was president of the Union.