The November deadline to restore devolution in Northern Ireland will be written into emergency legislation due to be brought before the British parliament within weeks, it emerged today.
But the Northern Secretary Peter Hain vowed the cut off point will not be extended if the parties fail to reach agreement.
The move came as Sinn Fein confirmed it will attend the reconvened assembly on May 15 with a view to restoring power sharing.
On the deadline set earlier this week by the Irish and British Governments, Mr Hain said: "It is rock solid firm.
"It will be in the emergency bill I will be introducing to Parliament soon after Easter so it will be in the statute books. "There is no shifting from it. "If everything collapses by then, then it will collapse. "There will be no coming back."
Mr Hain also told BBC Radio Ulster's Inside Politicsprogramme unionists should not fear any change to Northern Ireland's constitutional position if a deal on devolution cannot be achieved.
The Secretary of State said: "There's no question of joint authority or anything like that. "There's no question of joint government.
"That would be in contravention of the referendum that that the people of Northern Ireland voted on when they endorsed the Belfast Agreement.
"So there's no issue about that, it's not a constitutional matter." Prime Minister Tony Blair and Taoiseach Bertie Ahern last week set two deadlines for the formation of a multi-party executive as they unveiled their blueprint for restoring power-sharing at Stormont.
The first attempt to form a government in Northern Ireland will take place six weeks after the Assembly is recalled on May 15.
But with unionists, nationalists and republicans believing there is virtually no chance of an executive before the summer, realists believe the second deadline of November 24 will ultimately test whether politicians in Northern Ireland are up for reviving devolution.
Although Mr Blair and Mr Ahern did not outline how they would expand cross-border co-operation in the event of there being no power-sharing executive by November, it is understood both Governments have been looking at increasing the amount of cross- border bodies and areas of co-operation.
That could mean new, more dynamic bodies tackling health, the economy and education.
Stormont's 108 Assembly members were also warned on Thursday that if they did not reach agreement on the setting up of a government by November 24, their salaries and allowances would be stopped.
It was also confirmed there would be no Assembly election to a body which had not properly functioned since October 2002.