The Democratic Unionist Party's sole MEP has quit the party over its power-sharing deal with Gerry Adams, claiming Sinn Fein was still not fit for government.
It is the second time Jim Allister has resigned from the DUP.
In 1987 he resigned after he was prevented from standing for a Westminster seat because of an electoral pact with the Ulster Unionists.
But in 2004, he was persuaded back into frontline DUP politics when the Reverend Ian Paisley stood down from the European Parliament and topped the Northern Ireland-wide European election poll.
In his east Belfast office, the QC admitted that he had lost a battle within the DUP against the party striking a power-sharing deal with Sinn Fein. "It is with immense sadness that I must resign from the DUP," he declared.
"To continue as the DUP's MEP, it would be my obligation to accept the party executive policy decision to usher Sinn Fein into government in a few short weeks. This, in conscience, I cannot do. Thus, I must resign from the DUP. Sinn Fein, in my view, is not fit for government. Nor can it be in a few weeks."
It is believed the image of Mr Paisley and Mr Adams sitting side by side in Stormont's Parliament Buildings, sealing a deal which would see their parties form a power-sharing government on May 8, finally pushed the MEP over the edge.
Mr Allister expressed grave reservations about the prospect of devolved government featuring Sinn Fein, in the wake of last October's St Andrews talks.
He said: "I have fought a protracted battle within the party over recent months against a premature DUP/Sinn Fein government. I now have to accept that this battle is lost."
Mr Allister said he had believed the abolition of the IRA Army Council was always the litmus test of the Republican Movement's transition to exclusively peaceful and democratic means. He said if Sinn Fein believed the IRA was truly and irreversibly committed to exclusively peaceful means, there was no need for an Army Council.
