He suggested winning 200 seats - almost 30 fewer than in 2015 - would be a "successful" result for Mr Corbyn.
While many of Labour's policies are popular - among them, renationalizing some rail, energy and utility companies - the party faces hard questions about how it plans to fulfill its pledges without large increases in taxes and government borrowing.
Media captionJeremy Corbyn: "This is a manifesto for all generations".
Mr McCluskey's remarks will also be seen by Mr Corbyn's enemies in the party as evidence that the Labour leadership has all but given up hope of winning the election, despite claims that he is fighting to win.
"In terms of the imagery of Jeremy, that's a huge task", Politico quoted him as saying.
Merkel, Macron seek to revitalise ties to bolster EU
Dr Peter Ammon said there was "magic in the air" around Berlin as France's new president Emmanuel Macron met with Angela Merkel on Monday.
"But our votes are being hijacked by Tory and SNP politicians who want to use how we voted in referendums and general elections to assume something about the kind of future we want to see".
"I think there's a rumbling, a subterranean rumbling at the moment, where people are saying we want change".
Labour is the first of the major parties to publish its manifesto ahead of the June 8 general election, the BBC reported.
Mr McCluskey's prediction of just 200 seats for Labour and a big win for Theresa May and the Conservatives will be interpreted in two ways inside the Labour Party and by political opponents.
If Labour secured 200 seats it would be their worst result since 1935.
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The General Election could determine if the United Kingdom continues to exist, Kezia Dugdale warned, as she urged people in Scotland to reject the nationalist politics offered by the SNP and the Conservatives.
Both Gordon Brown and Ed Miliband resigned after leading Labour to defeats in 2010 and 2015 but, amid speculation that jockeying for position has already begun, there have been suggestions that Mr Corbyn could stay on if he equals the 30.4% vote share that Ed Miliband got in 2015.
KD: Labour offers a radical plan to change the country.
An analysis by the party of energy company accounts in 2013 found they were paying out £3.2 billion a year in dividends and interest payments.
A Conservative spokesman said: "Jeremy Corbyn can't deliver any of this - it just doesn't add up. These are made up numbers based on a shambolic manifesto with a £58 billion black hole at its heart". "While his figures are a fantasy, it is ordinary working families who will pay".
Polls open in first Iran presidential vote since atomic deal
Raisi also has promised monthly cash payments to the poor, a populist move that's been popular with Iranian voters in the past. That hasn't stopped those at Rouhani rallies from shouting for the house-arrested leaders of the 2009's Green Movement.
Its response was to say that 95 percent of earners would pay no more tax, but that the burden would start to rise on earnings of more than 80,000 pounds, about $103,000 at current exchange rates, and in corporate taxes.
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