Len McCluskey, the leader of the Unite union, has told Labour MPs "plotting" against Jeremy Corbyn's leadership to stop "scheming".
In a speech to the Oxford Union on Tuesday evening, which can be read in full on The Huffington Post, McCluskey attacked the so-called "moderates" in the Parliamentary Labour Party for refusing to accept Corbyn's victory.
"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 said.
McCluskey, who has previously warned the Labour leader not to just "say the first thing that comes into his head", acknowledged this evening that Corbyn's first few months in power had been characterised by some "sloppiness".
"Both Jeremy and his team are on a steep learning curve - but not in a classroom, instead on the front line and under heavy enemy fire," he said.
"From thirty years on the backbenches to having to carry out shadow cabinet reshuffles. It was never going to be easy. But if there has been some sloppiness in the early months of Corbyn's leadership that has not been the heart of the problem.
"The real difficulty has been the behaviour of a number of Labour MPs and Party grandees who have simply refused to accept the result of the leadership election.
"They spend their time both plotting behind the scenes, with every week producing another coup plan - or running to the media to attack the Leader and the policies that Party members voted for."
McCluskey said Labour MPs who were agitating against Corbyn had to be issued "a clear warning" not to try and force a leadership change.
He said the "bizarre" views of some, including former shadow cabinet minister Michael Dugher who has said May's local elections will be high-noon for Corbyn's leadership, "all have to be dismissed with distain by any real Labour supporter".
McCluskey said of so-called moderate Labour MPs: "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.
"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, against ID or against 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."
He added: "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."