Ed Miliband's showdown with Labour's biggest trade union backer has escalated after the party announced it was to hand evidence of alleged ballot-rigging to the police.
Party officials said they had contacted the Procurator Fiscal's Office in Scotland over claims Unite sought to pack the constituency party in Falkirk with its members in order to secure the selection of its favoured candidate for the safe Labour seat.
The move was angrily denounced by Unite General Secretary Len McCluskey who dismissed the allegations as "nonsense" and accused the Labour leader of deliberately seeking a "punch-up" with the unions.
The Tories meanwhile said Labour had been "bounced" into contacting prosecutors after Conservative backbencher Henry Smith wrote to the Chief Constable of Scotland asking him to investigate - a claim Labour strongly denied.
Mr Miliband insisted he was determined to uphold the integrity of the party and challenged Mr McCluskey to disown the activities of Unite in Falkirk.
"Instead of defending what happened in Falkirk, Len McCluskey should be facing up to his responsibilities. He should not be defending the machine politics involving bad practice and malpractice that went on there, he should be facing up to it," he said.
"We had members being signed up without their knowledge, bad practice, malpractice and, frankly, instead of defending that kind of thing, Len McCluskey should be condemning it."
The Unite leader said they would co-operate with the police inquiry but warned that it would not bring the matter to an end.
He said: "We've had shadow cabinet members saying that Unite have 'overstepped the mark'. What does that mean? We asked too many of our members to join the Labour Party. We should have told them that the Labour Party was full up perhaps? It is a nonsense."
Police Scotland meanwhile confirmed they had received the letter from Mr Smith. A spokeswoman said it was "currently being considered".