Speaking on the Andrew Marr programme, the General Secretary of Unite - one of Britain's biggest unions - urged the BBC to launch an investigation into the "sinister" company, Portland for its supposed role in the latest Labour leadership saga.
He said: "I’ll be honest: I’m amazed that some of the MPs have fallen into a trap. I think they’ve been seduced by sinister forces.
"If the BBC want to do an investigatory programme of a company called Portland, feel free to do so.
"This is a PR company with strong links to Tony Blair and right wing Labour MPs who’ve been involved in this orchestrated coup, and the coup has failed."
The conspiracy theory first appeared on pro-Corbyn website The Canary that outlined a series of loose connections and statements made by Portland employees and advisors.
It claims the company is orchestrating the coup through Blairite lobbyists with links to power players in the media.
While Corbyn fans have been avidly sharing the piece, it has come under much criticism from elswhere.
The Canary's strange, yet popular, theory goes like this: Portland was set up by Tony Blair’s former deputy communications chief Tim Allan. On its books are a number of Labour types, many of whom dislike Corbyn and also have links to the Fabian Society. The PR firm also has “countless links to the media” and the BBC recently interviewed a Portland consultant. Err, that’s it.
To be fair to [author of the article Steve] Topple, he is right to detect that Portland has a few active Blairites on the payroll. But on that basis, the entire British lobbying industry might also be behind Labour’s coup.
The PR company distanced itself from McCluskey's claims and asked that he withdraw his statement.
Elsewhere people were more scathing towards the theory...
People then began to suggest other things Portland could be behind.
Corbyn has insisted he is “ready to reach out” to his enemies in the party but warned he would stand for re-election if they staged a challenge for the top job.
He said MPs who have set their face against his tenure must “respect” the views of the members who elected him, reports the Press Association.
Top priority must be opposing the Conservatives and giving voters a real alternative, he said.
In an article for the Sunday Mirror, he wrote: “I am ready to reach out to Labour MPs who didn’t accept my election and oppose my leadership – and work with the whole party to provide the alternative the country needs.
“But they also need to respect the democracy of our party and the views of Labour’s membership, which has increased by more than 60,000 in the past week alone.
“Our priority must be to mobilise this incredible force to oppose the Tories, and ensure people in Britain have a real political alternative.
“That is my priority and always will be as leader of our party.
“Those who want to challenge my leadership are free to do so in a democratic contest, in which I will be a candidate.”
The plea for calm comes at the end of a tumultuous week that saw most of his front bench team walk out and 172 of his MPs back a motion of no confidence in his leadership.
Corbyn’s aides are reportedly keeping Tom Watson away from him, fearing he will try to “bully” the 67-year-old leader into quitting.
The deputy leader has been trying to seek a meeting to find a way of negotiating a settlement as the crisis engulfing the party shows no sign of abating.
Corbyn’s team said it had a “duty of care” to the leader and highlighted his age, according to the Observer.
A spokesman for Watson said: “Tom and Jeremy have always had a good working relationship and a friendly personal relationship. They have never even raised their voices to one another.”
Corbyn’s team said the claims were nonsense from an anonymous source. “Jeremy has met Tom on a couple of occasions this week,” a spokesman said.
Angela Eagle, who is tipped as a potential challenger to Corbyn, renewed her call for him to do the “right thing for the party and the country”.