The Northern Ireland assembly speaker Eileen Bell today ruled that the assembly link between the Ulster Unionist Party and the Progressive Unionist Party is contrary to the rules of the assembly.
Mrs Bell revealed after legal discussions that the Ulster Unionist Party Assembly Group does not have the characteristics of a party as laid down in the legislation governing parties.
Her ruling came as Ulster Unionist leader Sir Reg Empey endured growing internal dissent over his decision to bolster the ranks by bringing in PUP chief David Ervine.
Mr Ervine was astonished that Mrs Bell was able to take such a decision.
He said: "Here's a Speaker who can't call a meeting of the Northern Ireland Assembly and yet a Speaker who can make this type of judgment.
"I find it incredible."
Sinn Fein claimed the decision was not surprising, given that it contravened existing regulations.
But Naomi Long, deputy leader of the cross-community Alliance Party, claimed the UUP had been given a get-out clause.
She said: "Some leading members of the Ulster Unionists broke rank and expressed their disgust at this grubby link but clearly, the Ulster Unionists did not have the strength of character or morals to break this deal.
"Their blushes have only been spared by the decision of the Speaker to stop their disgusting deal."
Raymond McCord, an outspoken critic of Mr Ervine and the father of a UVF murder victim, was delighted by the ruling.
"Reg Empey was prepared to forsake principles, bringing in people from a party with a military wing.
"It's great news for me and a lot of other victims," he said.