The head of Labour's biggest trade union donor stepped up the bitter row with the party over alleged ballot-rigging, demanding Ed Miliband "step back from the brink of a ruinous division" and stop "playing into the hands" of the Tories.
Len McCluskey rejected any suggestions of criminal wrongdoing by Unite after the party called in the police over claims of irregularities in the selection process for its general election candidate in Falkirk.
Mr Miliband has accused the general secretary of the giant Unite union of defending "malpractice" and said he should "face up to his responsibilities" as he faces the stiffest test so far of his leadership.
With the Conservatives exploiting the row to accuse the Opposition of being controlled by trade union financial backers, Mr Miliband is expected to set out proposals within weeks to tackle "wider" issues raised by the controversy. But former London Mayor Ken Livingstone said he was personally assured by the party leader that would not include any move to end the historic formal links.
In a fresh assault on the party's handling of the Falkirk issue Mr McCluskey wrote in the Sunday Mirror that the union had been "working completely within the rules" to get a working-class candidate selected.
Dismissing an internal party investigation - which has not been made public - as a "shoddy fraud", he said Unite was happy to co-operate with any police probe but "reject the idea that any criminal wrongdoing has occurred".
The case centres on alleged attempts to stuff the Falkirk party with Unite members to ensure the success of its favoured candidate - some being signed up for Labour membership without their knowledge.
Mr McCluskey said he would not apologise for efforts to "reclaim Labour" which was "increasingly the preserve of an out-of-touch elite - Oxbridge-educated special advisers who glide from university to think tank to the green benches without ever sniffing the air of the real world".
He added: "Labour's attack on Unite is alienating tens of thousands of decent people from the party at a time when we should all be pulling together to take on this rotten government and its policies causing so much misery. So I urge Ed Miliband: step back from the brink of a ruinous division.
"Don't led Blairite diehards like Lord Mandelson be your back-seat drivers. They still haven't learned the lessons from the disappointments of New Labour. And don't let the Tories set our agenda. They are wrecking Britain. Their only hope is to wreck Labour too. This week the party's leaders have played into their hands. It is time to reunite."