DUBLIN — The fragile provincial government in Northern Ireland teetered on the brink on Wednesday evening after the arrest of a senior member of Sinn Fein, the Irish nationalist party that shares power with British unionists, in connection with a murder last month that the police have linked to Irish Republican Army operatives.

Peter Robinson, leader of the pro-British Democratic Unionist Party and first minister of the province, threatened to withdraw his party from the Northern Ireland Assembly, the provincial Parliament, within 24 hours unless its members voted to suspend business until the murder investigation is resolved or the British government did so unilaterally. The Democratic Unionists hold the biggest bloc of seats, and their withdrawal would lead to a government collapse.

“The D.U.P. has made it clear it will not be involved in business as usual,” Mr. Robinson said in a statement. Unionists accuse Sinn Fein, the I.R.A.’s former political wing and their partner in government, of breach of trust.

His ultimatum followed the arrest of Bobby Storey, a Sinn Fein leader and former I.R.A. commander, in the killing of Kevin McGuigan, 53, a former I.R.A. member and a father of nine who was shot outside his home in Belfast on Aug. 12. With the arrest of Mr. Storey and two other former I.R.A. members on Wednesday, 16 people have now been detained in connection with the killing, which the police say they believe was an act of revenge for the assassination of Gerard (Jock) Davison, reportedly a onetime Belfast commander of the I.R.A., in May.