Mr McCluskey - whose union is Labour’s biggest donors, giving around £7m since the start of last year - warned that failure to reinstate the former leader would leave a split party “doomed to defeat” at the next election.
His comment reflected widespread anger on the left of the party over the treatment of Corbyn, with former shadow chancellor John McDonnell branding his suspension “profoundly wrong” and Momentum denouncing it as “a massive attack on the left by the new leadership”.
In a sign of dissatisfaction with Sir Keir Starmer’s leadership, Unite earlier this month voted to cut its affiliation fees to Labour by 10 per cent, or just under £1 million.
Mr McCluskey made clear he believed political interference by Starmer lay behind the suspension.
"This was a day for our party to move forward as one to defeat the evil of antisemitism,” said the Unite boss, a member of Mr Corbyn’s inner circle and a close ally during his time in office.
”However, the decision to suspend Jeremy Corbyn has threatened that opportunity.
"The suspension appears to fly in the face of one of the important recommendations made by the Equality and Human Rights Commission - and which Keir himself said he would implement in full and immediately - which is to remove the leader's office from party investigations.
"But it is also an act of grave injustice which, if not reversed, will create chaos within the party and in doing so compromise Labour’s chances of a general election victory.
“A split party will be doomed to defeat.”
Mr McCluskey added: "I therefore call upon Keir to work across the party on a fitting and unifying way forward, to unite our party behind the implementation of the EHRC's important recommendations so that they can be taken forward as speedily as possible, and with the members' full trust and confidence.
"I also appeal to members angered by this suspension not to leave the party but to support moves to find a better way through. Working people are under fire like never before and ill-served by the worst government of our lifetimes. More than ever, they need a strong, united Labour party to stand up for them, ready to govern."