Progressive Unionist Party leader David Ervine is expected to join the Ulster Unionist group at the Northern Ireland Assembly.
David Ervine is to join the Ulster Unionist group at the assembly
It is understood an official approach was made to him on Friday, ahead of the assembly being recalled on Monday.
The PUP executive has held talks and Mr Ervine will remain its leader.
However, the calculated political move could mean the UUP receive an extra ministerial post, at the expense of Sinn Fein, if a government is formed.
The PUP's chairperson, Dawn Purvis, said the PUP took the decision after much consultation.
"It is felt that by forming a group, that will give the unionist community a much-needed boost," she said.
Ulster Unionist leader Sir Reg Empey said that for the first time since the assembly was established in 1998, unionism would "be punching its full weight".
"Unionists will be taking positions back from Sinn Fein. None of this will matter of course, unless the conditions are created that will lead to the restoration of devolution."
DUP deputy leader Peter Robinson said unionists had to be satisfied
Mr Ervine's move makes the Ulster Unionists the second largest party, after Ian Paisley's Democratic Unionist Party.
BBC correspondent John Thorne said the political consequences are complicated, and only matter inside an assembly that is not expected to reach any early agreement to restore power sharing.
But it could mean the Ulster Unionists merit an extra Executive post at Sinn Fein's expense, he added.
Sinn Fein's Mitchel McLaughlin has described the move as "breathtaking hypocrisy" on the part of Ulster Unionists.
"It is David Ervine's democratic right to join any group he wishes,"he said.
"But the UUP attempts to recruit David Ervine underlined unionist ambivalent towards loyalist violence in the starkest terms possible."
Northern Ireland's 108 assembly members will gather on Monday for the first time since they were elected in November 2003.
The political parties will then have six weeks to elect an executive.
If the parties fail to do so, they will get a further 12 weeks to try to form a multi-party devolved government. It that attempt fails, their salaries will be stopped.
The NI Assembly is being recalled on Monday
The British and Irish governments would then work on partnership arrangements to implement the Good Friday Agreement.
Meanwhile, the deputy leader of the DUP, Peter Robinson, has said his party's consultation of the unionist community on whether to share power with republicans may begin in the autumn.
But he said the Independent Monitoring Commission would need to conclude that the IRA had ended all paramilitary and criminal activity.
"We are on the ground so we have a fair idea what people's thinking is," he told BBC Radio Ulster's Inside Politics programme.
"We want to give them that wider opportunity, that the prime minister flagged up, that people have to be satisfied.
"We will give them that opportunity, presumably some time around October or November."
Devolved government at Stormont was suspended in October 2002 following allegations of a republican spy ring.
A court case arising from the allegations later collapsed.