Sir Robin Chichester-Clark, who has died aged 88, was the epitome of the “big house” unionists who dominated political and public life in Northern Ireland in the decades after partition in 1922. He represented Londonderry in the House of Commons from 1955 until 1974 and earned the distinction of being the only member of his party to serve at any ministerial level in a British government. While in opposition he held several shadow posts under Edward Heath, who appointed him minister of state for employment in his 1970 administration.

Until 1966, the Ulster Unionists held all 12 Northern Ireland seats at Westminster but the election of the nationalist Gerry Fitt in West Belfast that year fractured the longstanding monopoly and signalled the beginning of a new and more turbulent era for unionism.

Since partition, there had been a “convention” that Northern Ireland affairs were not discussed in London, an arrangement that suited Whitehall ministers and officials wary of any contact that might ignite the ever-glowing embers of crisis in Belfast.

But, with the private encouragement of the Labour prime minister, Harold Wilson, and the vociferous support of a cadre of Labour MPs formed into the Campaign for Democracy in Ulster, Fitt challenged the prevailing order, raising long-held grievances about voting rights, religious discrimination and other contentious issues.

As leader of the unionist MPs it fell to the innately moderate Chichester-Clark to defend his party’s partisan record. Having lost the fight to save the convention, he and his colleagues secretly consulted the RUC inspector-general, seeking material from special branch to discredit Fitt, whom they had come to regard as an IRA agitator. In the end Chichester-Clark appeared quite a woeful figure in the Commons when he could only cite Fitt’s preference for an Irish passport as evidence of his subversive leanings.

When the campaign for civil rights in Northern Ireland accelerated after a violent encounter between marchers and the RUC in October 1968, Chichester-Clark, as the Conservative spokesman on NI affairs, had the impossible task of defending the increasingly repressive security policies being pursued in Belfast and countering the universal scorn of the Commons, where the need for urgent and far-reaching reform at Stormont was now accepted on both sides of the house.

Given the traditional unionist links with the Conservative party, Chichester-Clark was first disappointed and then disillusioned as even the moderate unionist case for gradual reform he advocated was comprehensively rejected. His hurt was all the more acute because his older brother, James, the Stormont prime minister between 1969 and 1971, later made a life peer as Lord Moyola, was struggling to advance cautious reform against the stridency of his uncompromising party hardliners and a backdrop of intensifying civil disorder.

When Heath became prime minister in 1970 and refused to ease the pressure for reform at Stormont, Chichester-Clark’s credibility and influence was further undermined

After Stormont was prorogued in favour of direct rule from Westminster in 1972, and the government held fruitless talks with the IRA in London, he felt completely betrayed. When his government then entered into the Sunningdale agreement with the Irish Republic in 1973, his disillusion was complete.

With discontent growing among his constituency party members in Northern Ireland, Chichester-Clark decided to get out before he was disowned, but his failure to secure an alternative Conservative candidacy in Britain to contest the 1974 general election finally ended his political life. He was knighted later that year.

Named Robert, but known as Robin, he was born at Moyola Park, the family’s sprawling ancestral estate at Castledawson, Co Derry, at the foot of the Glenshane Pass. His father, James Lenox-Conyingham Clark and mother, Marion (nee Chichester), had changed their names by deed poll in 1924 to the simpler Chichester-Clark. He had two siblings, James and Penelope.

Robin was initially educated at home by governesses but later was sent to Selwyn House prep school near Broadstairs, Kent. For part of his time there, the school was evacuated to Wales. When he left in 1940 he enrolled at the Royal Naval College, Dartmouth, before reading law and history at Magdalene College, Cambridge.

He first worked in journalism then public relations, representing Glyndebourne Opera in 1952-53. He joined the Oxford University Press, where he remained until, following in the footsteps of his father and grandfather, becoming the MP for Londonderry in 1955.

After leaving politics, Chichester-Clark set up as a management consultant and was active at board level with a broad range of organisations, both commercial and artistic. He was also involved in fundraising for the Royal Philharmonic Orchestra.

He is survived by his second wife, Caroline (nee Bull), whom he married in 1974, and their two sons, Adam and Thomas; and by three children, Emma, Mark and Sophia, from his first marriage, to Jane (nee Goddard), which ended in divorce.

• Robert (Robin) Chichester-Clark, politician, born 10 January 1928; died 5 August 2016