Labour is on course for its worst defeat since 1935, according to the general secretary of Unite.
Key Corbynista Len McCluskey, who has donated millions of pounds to Labour's campaign, said his party will fail to defeat Theresa May, but he would be happy if she did not win a landslide.
However, he said Jeremy Corbyn would be lucky if he held on to 200 seats after June 8th.
In an interview with Politico, Mr. McCluskey said Mr. Corbyn faces an immense task and it would be extraordinary if he won next month.
If the Unite general secretary's predictions are accurate, this would leave the Conservatives with a majority of 80, an increase from their 12-seat majority.
Labour currently has 229 seats.
But Mr. McCluskey did not blame the Labour leader for this anticipated outcome. Instead, he piled responsibility on the press for pinning a particular image of Mr. Corbyn.
'Huge task in reviving Mr. Corbyn's image'
He said there is a huge task facing the party in reviving the Labour leader's image.
The Unite general secretary said Mr. Corbyn is a decent and honest man who is on people's side.
He added that his union is helping Labour to work hard at pumping out their message of an equal and fairer Britain.
But Mr. McCluskey believes that because of the large crowds that turned up to Labour's manifesto launch yesterday, the polls may well be wrong and there is all to play for.
'A devastating blow'
Despite this, the Unite general secretary's comments will come as a devastating blow to Mr. Corbyn's manifesto launch.
The manifesto outlines the Labour leader's 'radical and responsible' plans that aimed to reverse national priorities.
He ruled out VAT and national insurance rises and promised to implement a new living wage rate of £10 per hour.
Mr. Corbyn said he aims to provide hope and opportunity for everybody.
He told the crowd that gathered in Bradford he believes British people are moving back towards Labour.
Other details include creating nine new public bodies to run the water and sewage system in England, which is currently privatised.
He aims to end the practice of rewarding dividends to households and to save families £100 a year in energy bills.
Labour also intend to provide families with children aged two to four with 30 hours free childcare.
'Ensure the nation lives within its means'
Mr. Corbyn wants to eradicate austerity whilst ensuring the nation lives within its means.
But a draft version of the document was leaked last week, which revealed Labour intends to renationalise railways, the Royal Mail and electricity and transmission distribution networks.
Mr. McCluskey's remarks were not the first distraction from Labour's campaign message.
The Labour leader said the costs behind many of his policies had been checked before they were featured in the manifesto.
But the shadow chancellor, John McDonnell, contradicted his leader's message.
Guido Fawkes reported that the shadow chancellor embarrassed Mr.
Corbyn earlier on yesterday morning.
It appeared Mr. McDonnell was handed a note in the middle of an interview with BBC political correspondent, Nick Robinson, which stated how much the UK's budget deficit currently is as he was unable to answer Mr. Robinson's question about it.
Both the shadow home secretary, Diane Abbott, and the shadow education secretary, Angela Rayner, failed to clarify specific numbers regarding the number of policemen and schools in England respectively during live interviews.
The Conservatives responded to Labour's election errors so far by saying taxpayers would have to foot the party's unfunded spending commitments.
Treasury minister David Gauke described the party's economic ideas as 'nonsensical', and said they would have to raise taxes dramatically because their sums do not add up.
Len McCluskey has been general secretary of Unite since 2011.
He resigned in December 2016 to fight this year's leadership election.
He was re-elected as general secretary last month with 59,067 votes, beating his rival, Gerard Coyne, who won 53,544 votes.
Mr. McCluskey has recently appeared in the news for falling down the stairs of the building where Labour's manifesto was signed.
