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The boss of Britain's biggest union has claimed the coup on Jeremy Corbyn is being "orchestrated" by a "Blairite" PR firm.

Len McCluskey told the BBC to mount an investigation into Portland Communications as he claimed MPs have been "seduced by sinister forces in play".

The Unite general secretary was referring to a comment piece by 'left-wing alternative' website The Canary which claimed "it surely can be no coincidence" Portland employees have ties to centrist Labour groups.

The website appears to have provided no hard evidence of a Portland-run coup, relying instead on anti-Corbyn statements by its employees and advisors, who include ex-Blair spinner Alastair Campbell.

Meanwhile 64 members of Labour's frontbench team, nearly two thirds of all of them including long-time Corbyn friend Andy Slaughter, have resigned their posts.

And Portland tweeted at Mr McCluskey: "This is a ridiculous conspiracy theory and completely untrue."

But Mr McCluskey told the BBC's Andrew Marr Show: "To be honest I'm amazed that some of the MPs have fallen into a trap.

(Image: Rick Findler/PA Wire)

"I think they've been seduced by sinister forces in play here. If the BBC want to do an investigatory programme of a company called Portland then feel free to do so.

"This is a PR company with strong links to Tony Blair , right-wing Labour MPs, who have been critically involved in this orchestrated coup."

Supporters rushed to congratulate Mr McCluskey for airing the theory on the BBC, while Labour commentator James Bloodworth attacked the "conspiracy theory" as "full lizard".

David Singleton, editor of Public Affairs News, called the theory "strange", accusing the piece of missing out Portland's Tory-linked employees or other pressure groups.

He wrote in the New Statesman: "Since the mid-1990s, Labour lobbyists have tended to come from the pragmatic, Blairite ranks of the party. This is largely because Labour spent the 1980s ignoring business, and that only changed significantly when Blair arrived on the scene."

Mr McCluskey today added it was "unhelpful for ex leaders to be wheeled out" against Mr Corbyn, saying Ed Miliband, Neil Kinnock and Gordon Brown all lost elections.

He said: "Grandees being dragged out to be part of this unedifying coup is quite outrageous.

"This has been a political lynching of a decent man undermined, humiliated, attacked in order to push him out.

"And here's the truth. It's failed. The coup has failed."