The leader of Britain’s biggest union has accused a public relations company of being the “sinister” mastermind behind the current crisis engulfing the Labour leadership.

Jeremy Corbyn is expected to face a leadership challenge in the coming week after Labour MPs voted by 172-40 last week that they have no confidence in him. He has defied calls to resign from former shadow cabinet colleagues, and several ex-leaders of the Labour Party including Ed Miliband.

The latest call came from Neil Kinnock, who led Labour from 1983 to 1992. He told the BBC’s Andrew Marr Show that there is “no basis on which Jeremy really could or should stay” after losing the support of his fellow MPs.

Dave Brown on Jeremy Corbyn Show all 12 1 /12 Dave Brown on Jeremy Corbyn Dave Brown on Jeremy Corbyn Dave Brown Dave Brown on Jeremy Corbyn Dave Brown Dave Brown on Jeremy Corbyn Dave Brown Dave Brown on Jeremy Corbyn Dave Brown Dave Brown on Jeremy Corbyn Corbyn's reshuffle Dave Brown on Jeremy Corbyn Corbyn and the Syria bombing vote Dave Brown on Jeremy Corbyn Corbyn asks questions from the public at PMQs, meanwhile backbenchers plot to oust him Dave Brown on Jeremy Corbyn Corbyn is unavailable to attend the Privy Council Dave Brown on Jeremy Corbyn Conference rejects Corbyn’s call to debate Trident Dave Brown on Jeremy Corbyn At Labour conference Corbyn and McDonnell press for a Robin Hood tax Dave Brown on Jeremy Corbyn Corbyn’s hopes for a ‘new politics’ look optimistic in the face of a media barrage Dave Brown on Jeremy Corbyn Corbyn enters Labour leadership race

But the general secretary of the Unite union, which helped Jeremy Corbyn win his leadership victory last year, claimed that left wing MPs who have joined the call for Mr Corbyn's resignation have been manipulated by Portland Communications, a public relations company set up by Tim Allan, who previously worked under Tony Blair as a Downing Street spin doctor.

During his appearance on the Marr programme, Len McCluskey did not give any evidence to support the allegation, which Mr Allan claimed is “completely untrue.”

Mr McCluskey 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.”

A tweet from Portland Communications demanded that he retract the allegation, saying: “This is a ridiculous conspiracy theory and completely untrue. He should withdraw.”

Mr McCluskey also brushed aside the calls from ex-leaders for Mr Corbyn to go. He said: “It’s not helpful for ex-leaders to be wheeled out . Neil (Kinnock) is a nice man but the reality is he was leader of the Labour Party for eight years and lost two elections; Ed Miliband, leader of the Labour Party five years, and lost an election; Gordon Brown – 13 years at the top of our party – lost an election. Grandees being dragged out to be part of this unedifying coup is quite outrageous.”

Watson on Corbyn