James Molyneaux, the soldier who helped liberate Bergen-Belsen concentration camp and later led the Ulster Unionist party through some of the darkest years of Northern Ireland’s Troubles, has died.

The 94-year-old life peer, who took the title Lord Molyneaux of Killead in 1997, was leader of the UUP from 1979 to 1995, having joined the party a year after the second world war.

A passionate unionist who believed Northern Ireland would be better off fully integrated into the UK, Molyneaux represented the constituency of South Antrim in the House of Commons for more than two decades.

He was bitterly opposed to the 1985 Anglo-Irish agreement, which gave Dublin a say in the running of Northern Ireland’s affairs.

Alongside his one-time unionist opponent Ian Paisley, he denounced Margaret Thatcher as a traitor for signing the accord with the Irish government in front of tens of thousands of unionists at Belfast City Hall. Previously Paisley had described Molyneaux as a “Judas Iscariot” figure who could not be relied upon.

The former MP also opposed the Good Friday agreement in 1998 and became a thorn in the side of his successor, David Trimble, who had argued that the deal hatched in Easter week that year would secure the union.

Molyneaux was an intensely private man who never married, but in a documentary to mark the 56th anniversary of Bergen-Belsen’s liberation, he spoke movingly of his experience being among the RAF’s first medical teams arriving in the concentration camp.

The current UUP leader, Mike Nesbitt, paid tribute to his predecessor. “It is a sad day because although he has had a long life, and a life well lived, it is still extremely sad that the day has come when he is no longer with us.

“He was a towering figure right through some of the most difficult days that Northern Ireland has ever seen and he is a man that this party – the Ulster Unionist party – owes a huge debt of gratitude to.”

Lord Empey, his Ulster Unionist colleague in the House of Lords, also expressed his condolences.

He described Molyneaux as a “calming influence during the hunger strikes, the Ango-Irish agreement and through to the IRA and loyalist ceasefires.”

David Ford, Northern Ireland’s justice minister and leader of the centrist Alliance party, also paid tribute to the veteran unionist.

Ford said: “He was a hard-working public representative who always endeavoured to address the concerns of his constituents.”