BELFAST, Northern Ireland - Northern Ireland will elect a legislature March 7 that will determine the fate of power-sharing, Britain announced yesterday in swift reply to an IRA-linked political party's decision to support the police.

The prime ministers of Britain and Ireland, Tony Blair and Bertie Ahern, said they hoped a new Assembly would produce what its predecessor failed to achieve - a coalition that unites Protestant hard-liners with Irish Republican Army veterans.

Both prime ministers warned the two biggest parties in the British territory - the Protestants of the Democratic Unionists and the Catholics of the IRA-allied Sinn Fein - this would be their last chance to cooperate. Previous deadlines have passed without consequences.

Speaking after a London meeting, Blair and Ahern said the new Assembly must form a 12-member cabinet within a week after the election, and be ready to receive control of most Northern Ireland government departments by March 26. Failure would mean the Assembly's abolition the next day.

Power-sharing was the central goal of the Good Friday peace pact of 1998, but a four-party coalition collapsed in 2002 amid incessant Protestant-Sinn Fein arguments over the IRA's future. Since then, Democratic Unionist leader Ian Paisley has stressed he will cooperate only after Sinn Fein accepts British law and order.

Sinn Fein took a big step Sunday when a party conference voted to begin cooperating with the Police Service of Northern Ireland, which is predominantly Protestant. A report released yesterday also found the IRA to be complying fully with its July 2005 promise to renounce violence for political purposes.