The leader of the Labour's biggest financial backer, Unite, has said the party will not win the election on 8 June.
Len McCluskey said that if Labour manages to keep 200 of its 230 seats, that would constitute success. Winning 200 seats would be Labour's worst result since 1935.
McCluskey told the Politico website: "Labour's policies will make Britain a better and more equal society. The scale of the task is immense. People like me are always optimistic ... things can happen. But I don't see Labour winning. I think it would be extraordinary.
"I believe that if Labour can hold on to 200 seats or so it will be a successful campaign. It will mean that Theresa May will have had an election, will have increased her majority but not dramatically," he said.
Tuesday saw the launch of Labour's manifesto which set out pledges to spend £50bn and increase taxes for those earning over £80,000.
But Carl Emmerson, deputy director of the IFS, said how the higher taxes would impact the public finances was uncertain, telling the Guardian: "They will almost certainly bring in tens of billions of pounds but quite possibly not the full amount that Labour expects."
Meanwhile, Carolyn Fairbairn, director-general of the CBI business lobby group, told the FT: "Labour's proposals taken as a whole prioritise state intervention over enterprise, and fail to offer the pro-growth and competitiveness agenda the country so badly needs."
Corbyn has said he would stay as party leader whether he won or lost and the BBC has reported that he could remain at the helm if he equals the 30.4% vote share that Ed Miliband got in 2015.
On Wednesday (17 May), the Liberal Democrats will release their manifesto in which they will promise a second Brexit vote, financial help to get on the housing ladder and cheaper bus fares. The Green Party will also launch its manifesto, and says it will provide free sanitary products for women in financial need.