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Peter Robinson, who heads the pro-British Democratic Unionist Party and is the first minister of Northern Ireland, resigned from the assembly after an adjournment motion failed.Credit Charles Mcquillan/Getty Images

DUBLIN — The fractious power-sharing government in Northern Ireland effectively collapsed on Thursday as Peter Robinson, leader of the majority party, followed through on his threat to resign as the province’s chief executive. It was the latest chapter in a political crisis prompted by claims from the police that Irish Republican Army operatives were involved in the murder of a former member last month.

Mr. Robinson, who heads the pro-British Democratic Unionist Party, said all but one of his party’s members would follow him in stepping down from the Northern Ireland Assembly, the provincial parliament. The move came after other parties refused to vote to suspend assembly business pending an investigation into the allegations, as Mr. Robinson, who had been first minister of Northern Ireland, had demanded.

Mr. Robinson said the assembly needed to adjourn to discuss the fallout from the killing of Kevin McGuigan, a former I.R.A. member, on Aug. 12 and the subsequent arrest of senior members of Sinn Fein, the majority Irish nationalist party in the assembly. Last week, the minority Ulster Unionist Party resigned over the issue, citing distrust of Sinn Fein, once the political wing of the I.R.A.

The crisis deepened on Wednesday after the arrest of three former senior I.R.A. figures, including Bobby Storey, a leader of Sinn Fein who is widely regarded as a close confidant of its president, Gerry Adams.

The failure to reach agreement on an adjournment creates a virtual suspension of the multiparty government structures that have been in place in their current form since 2007. The assembly, which was created as a result of the 1998 Good Friday peace agreement, has been suspended several times before, sometimes for several years. On those occasions, Northern Ireland reverted to direct rule from London.

Mr. Robinson said one Democratic Unionist, Arlene Foster, would remain in the assembly as acting first minister “to ensure that nationalists and Republicans are not able to take financial and other decisions that may be detrimental to Northern Ireland.”

Alasdair McDonnell, leader of the minority nationalist Social Democratic and Labour Party, said it voted against the motion to adjourn because party members did not believe that it would have bought the necessary time.

“Adjournment would not have added anything,” he told journalists outside the assembly building after the vote. “When the adjournment was over we would still have been drifting toward suspension. The adjournment was not the solution, and we looked at this long and hard.”

At a news conference after the vote, Mr. Adams, the Sinn Fein president, said it should not be blamed for the murder of Mr. McGuigan, who is believed to have been killed in revenge for the murder in May of another former I.R.A. activist, Gerard (Jock) Davison.

“We are not responsible for criminal elements out there,” he said. “Those who are against equality, harmony and peace and those who killed Kevin McGuigan are the only ones who will benefit by bringing down these institutions.”

A statement from the British government said Prime Minister David Cameron was “gravely concerned” about the situation and would call Mr. Robinson and the Northern Ireland secretary, Theresa Villiers, to discuss it.