The comments - in a piece for HuffPost UK - are highly significant as Unite is Labour's biggest financial backer and the general secretary has previously been a staunch ally of Mr Corbyn.
It comes after Labour lost 60 seats in its worst election performance since 1935.
Mr Corbyn has vowed to quit after a 'period of reflection', and earlier on Friday said: 'The responsible thing to do is not to walk away from the whole thing, and I will not do that.'
Mr McCluskey reserved the majority of his criticism for senior Labour figires who had advocated for a pro-Remain position and led the party into a 'slow-motion collapse into the arms of the People’s Vote movement'.
He said: It is pretty obvious where the essential reason for Thursday’s hugely disappointing result can be found.
'When our losses are concentrated in former coalfield constituencies and other post-industrial communities that voted heavily "Leave" in the 2016 referendum, and yet we happily retain our position in London more-or-less unscathed, it is staring us in the face.
'Others will try to make a different case, either because they have volubly hankered after the New Labour past throughout the years of Corbyn’s leadership of the party, or because they lack the honesty to accept the consequences of their advocacy of keeping Britain in the EU at any political price.'
But he also acknowledged 'mistakes' made by the party's leadership throughout its campaign, including what he called an 'incontinent rush of policies which appeared to offer everything to everyone immediately'.
Taking direct aim at the Labour leader, Mr McCluskey - who has previously backed the party's handling of anti-Jewish abuse - said Mr Corbyn's 'failure to apologise for anti-Semitism in the party when pressed to do so' had capped 'years of mishandling of this question'.
While he said Labour's Brexit position, backed at its party conference earlier this year, was 'the right and honourable one' he said it had been 'fatally undermined from the outset by leading members of the shadow cabinet rushing to the TV cameras' to promise to back Remain in a future referendum.
The Unite chief said: 'Both Labour’s target seats, and the ones most at risk in the north and the Midlands, were preponderantly in Leave-voting areas with very small Liberal Democrat and Green votes. Put bluntly, there were far more coalfield seats to lose than there were Canterburys to win.
'As it is, a year of worrying about and placating exclusively Remain voters has produced the backlash which some of us predicted. Better by far that we had stuck with some updated variation of the 2017 Brexit position, rather than its negation.'
Urging the party to 'rebuild, reflect on what went wrong and inevitably elect a new leader early in the near future', Mr McCluskey said: 'Corbyn has borne the brunt of one of the most sustained and unpleasant character assassinations in political history and done so with dignity.
'But alas some of the mud stuck and his leadership became an issue on the doorstep.'
And, in a thinly-veiled dig at Mr Corbyn, he said: 'The next leader needs to understand the communities that gave birth to the Labour movement, and realise that the whole country is not very like Labour London.
'As important as it is, too often, Labour addresses the metropolitan wing of its electoral coalition in terms of values – openness, tolerance, human rights – and the "traditional" working-class wing simply in terms of a material offer, as if their constituencies did not have their own values of solidarity and community. That must change.'
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