THE head of Britain’s biggest union hit social media last night to show his “full support” for Jeremy Corbyn hours after suggesting the Labour leader would step down before the general election if the party’s prospects were “still awful”.

Unite boss Len McCluskey, who has been a powerful and crucial ally of Jeremy Corbyn, said earlier in an interview: “Let’s suppose we are not having a snap election. It buys into this question of what happens if we get to 2019 and opinion polls are still awful.

"The truth is everybody would examine that situation, including Jeremy Corbyn and John McDonnell.”

He said Corbyn and McDonnell were “not egomaniacs” and would not try to “cling on” at the head of the party.

McCluskey also warned that Labour needed to sort out its position on immigration and “get its narrative right on free movement” in order to avoid losing ground to Ukip.

He said: “We need to expose what Ukip stand for: not just anti-foreign but anti-workers’ rights, their hidden agenda on the NHS and welfare state. But it will only work if ordinary people believe Labour is listening to their concerns and has solutions.”

His comments triggered a bitter exchange with his rival for the Unite leadership, Gerard Coyne, who claimed that the union chief was acting as Labour’s “puppet master”.

Coyne said: “I am astonished and deeply concerned that, at a time like this, Len McCluskey should deliver what amounts to a public ultimatum to the leader of the Labour Party.

“My criticism of his handling of the role of general secretary of Unite is not whether he has backed the right leader or the wrong leader of the Labour Party, but that he appears to think it is his job to be Labour’s puppet master.

“In 2015 and in 2016, he decided the Labour Party should be led by Jeremy Corbyn, and spent hundreds of thousands of pounds of Unite members’ money to make that happen. It is not in the interest of Unite’s members that the general secretary should spend so much of his time and their money playing politics.”

McCluskey used a series of social media posts to say the Labour leader was a “genuine, decent man fighting for a fairer Britain” and shot back at Coyne, claiming his campaign was being orchestrated by the “failed plotters” who had sought to oust Corbyn. “He [Coyne] claims that I decided that Unite should support Jeremy Corbyn as Labour leader in 2015 and 2016, yet the truth, as he is well aware, is that in 2015 this was a decision of our elected lay executive council, and in 2016 of our 600-strong policy conference, by a vast majority. To claim otherwise is to disrespect our membership and our democracy, while asserting that our union is a political ‘puppet master’ panders to the worst anti-Labour stereotypes of the media.

“These unscrupulous remarks show that Gerard Coyne’s campaign is not being driven by concern for Unite and its members’ interests.

“It is being scripted by the failed plotters in the Parliamentary Labour Party, for whom Unite would be collateral damage in their political project to bring back Blairism.”

Unite, which gives millions of pounds a year to Labour, was one of Corbyn’s strongest supporters in last summer’s contest.