Unite boss Len McCluskey tonight attacked “disloyal” Labour MPs who oppose Jeremy Corbyn and urged them to focus their fire on the Tory Party.

The union General Secretary claimed that Corbyn reflected the views “of most Labour supporters” on many of the 500 occasions on which he broke the party whip.

And he said his message for “the plotters” is clear: “stop the sniping, stop the scheming, get behind Jeremy Corbyn and start taking the fight to the Tories.”

Speaking at the Oxford Union McCluskey said he is not in favour of mandatory re-selection but said a “clear warning” must be sent to Corbyn’s critics.

McCluskey used his appearance to attack the perceived lack of “big ideas” from Corbyn’s rivals in last summer’s leadership contest, as well questioning the labelling of some backbenchers as “moderates”.

“There is nothing ‘moderate’ about voting to bomb Syria or agreeing more public spending cuts, anything more than it’s ‘extreme’ to vote for peace or for an end to eye-watering austerity,” he said.

In one of his biggest public shows of support for Corbyn to date, McCluskey said:

“Some have sought to excuse their disloyalty to Corbyn by pointing to his own rebellious past on the backbenches. But who can seriously argue that his votes in parliament against the Iraq war, identity cards or university tuition fees now diminish his ability to lead the Labour party today? On all these issues he was not only right, I believe, he was giving voice to the views of most Labour supporters.

“I’m not saying that any Labour MP should have to abandon his or her own views, or cease to articulate them within the party’s democratic structures. But I am saying that this continual war of attrition is achieving nothing beyond taking the pressure off the government.

“So my clear message to the plotters is – stop the sniping, stop the scheming, get behind Jeremy Corbyn and start taking the fight to the Tories.”

Unite, which represents 1.4 million workers across the country, backed Corbyn in the leadership race, who ultimately won with more than half of all votes from trade unionists.

“These MPs, who refuse to accept the overwhelming mandate Jeremy Corbyn got from Labour’s membership, are generously described as the ‘moderates’ in the party. It’s an abuse of language,” McCluskey said.

He will add: “Such labelling simply obstructs the debate we need to have which is what went wrong with New Labour, what lessons can we learn, and how can we craft an appealing electoral pitch for the reality of 2020, not 1997?

“Their analysis of Labour’s defeat in 2015 was unconvincing, their proposals stale, minimalist and uninspiring – and for the most part they have still not shaped up after Corbyn’s victory. Until they can do that, they are a plot without a programme; a cabal without a critique.”

He also opposed making it easier to deselect sitting Labour – but his belief that a “clear warning” is needed to anti-Corbyn MPs will do little to ease nerves within the Parliamentary Labour Party.