Union boss Len McCluskey has accused deputy Labour leader Tom Watson of being involved in a world of "skulduggery, smears and secret plots".
He hit back after Mr Watson warned the party's future was at risk from hard-left supporters of leader Jeremy Corbyn, who he warned appeared to be plotting a "secret deal" with Mr McCluskey.
In a hard-hitting blog for The Huffington Post UK, the Unite leader said he had been touring the country in recent months meeting "decent" workers as he campaigns to be re-elected general secretary.
The workers are worried about their jobs, Brexit and public services, said Mr McCluskey, adding there was "another world" in the Labour movement.
"A world of skulduggery, smears and secret plots. That is where you will find Tom Watson.
"When Labour has needed loyalty, he has been sharpening his knife looking for a back to stab. When unity is required, he manufactures division," he wrote.
Mr McCluskey said he had been accused of being "secretly in cahoots" with Jon Lansman, founder of Momentum, to bankroll the grassroots activists group which helped propel Mr Corbyn to the leadership.
"That was at breakfast. By lunchtime I was no longer the scheming mastermind, but Unite chief of staff Andrew Murray was doing the conspiring instead.
"By evening, he may well have been blaming the Unite caretaker for digging a secret tunnel linking Unite HQ to Jon Lansman's home.
"The fact is that I have never had a private meeting with Lansman about anything in my life, let alone on this alleged scheme."
Only Unite's executive could decide to support such an organisation, and there was "no proposal that it should do so".
Mr McCluskey added: "Why the untruth? Watson wants to control the Unite election. He is not ready to leave the matter in the hands of the ordinary working people who make up our union.
"He has been masterminding a nasty tabloid-style campaign against me - standard fare in political elections, alas, but something new for the trade union movement.
"His transparent plan is to install a puppet who can take the union back to the bad old days when unions like the one he worked for were in the pockets of employers and failed to put their members' interests first."
On Monday night, Mr Corbyn made a fresh plea for unity and acknowledged that "spirits can run high" because Labour is a "passionate party".