The starting gun has been fired in the battle to lead Labour's biggest trade union backer - in a vitally important contest for Sir Keir Starmer.
In an emotional statement opening a meeting of the council, Mr McCluskey - the firebrand left-winger known as "Red Len" - described his impending retirement as "a bitter-sweet moment".
Four candidates are expected to enter the race: assistant general secretaries Steve Turner and Howard Beckett, executive officer Sharon Graham and Gerard Coyne, narrowly defeated by Mr McCluskey in 2017.
Announcing the election launch, a Unite source told Sky News: "After a full discussion by our executive council, it has been agreed that Unite's general secretary election is under way.
"A timetable will be circulated once agreed with the independent scrutineer. The election will be concluded by the end of August."
Sky News understands that subject to confirmation by the scrutineer the election timetable will be:
Nominations will close on 11 June
The election will take place between 12-23 July
Votes will be counted on 24 and 25 August
The result will be declared on 26 August
In a move which opponents of Mr McCluskey claim is a "stitch-up" by the union leadership, candidates will require 174 nominations, a big increase from 50 when Mr Coyne challenged Mr McCluskey five years ago.
The result of the election will be crucial for the Labour leader, who will be hoping to boost his leadership and tighten his grip on the party with the help of more friendly union leaders.
Mr McCluskey, 70, was Jeremy Corbyn's chief union cheerleader in the unions and has claimed Sir Keir is turning Labour into "the party of the establishment " and is heading for "the dustbin of history".
Unite is Labour's biggest financial backer, donating £3m at the last election, though last October the union cut its affiliation by 10%, in a move thought to reduce its funding by almost £1m.
The leadership election also comes at a turbulent time for the union, with Mr McCluskey facing calls for an inquiry over the £98m spent on a hotel and conference centre in Birmingham.
Of the four expected candidates, Mr Turner, Mr Beckett and Ms Graham are all considered left-wingers, while Mr Coyne, a former West Midlands regional secretary and Starmer backer, is supported by leading right-wing Labour MPs.
Declaring his candidacy, Mr Turner, who began his working life as a London bus conductor, said he had played a leading role in negotiating the furlough scheme with the Treasury last year and protecting millions of workers during the pandemic.
He told Sky News: "For 39 years I've walked in the shoes of our members; listening, learning and leading - from shop steward and officer to assistant general secretary.
"I've been unemployed and know what it's like to lose your job, struggling to pay bills and put food on the table.
"It's my life experiences that drive me, that motivate me to be the best I can be, from helping secure the furlough scheme, to leading the charge in the fight for the jobs of today and the livelihoods of tomorrow.
"We will not allow the pandemic to be exploited by opportunistic bosses, or workers' wages pinched to boost the riches of profitable companies and line the pockets of multimillionaires."
Launching his campaign, Mr Coyne - who already has more than 200 nominations, according to his supporters - told Sky News: "Unite members and reps are crying out for change, not more of the same.
"Unite has spent nearly £100m building a hotel and conference centre in Birmingham. That's a clear example of the wrong priorities and the wrong approach.
"I will use the union's resources and energy to empower our workplace reps and support our members through the troubled times to come - not to play political games or waste their hard-earned money.
"I believe that Unite members should know their union has got their back. As general secretary, I will stand alongside every member of Unite as they face a future of uncertainty. The union I lead will be run for its members - always available, and always strong."
Mr Beckett has been highly critical of Sir Keir's leadership. Last year he led a walkout by left-wingers at a meeting of Labour's national executive in protest at the leader's "factionalism".
Confirming that he is running, Mr Beckett said: "This is a critical time for workers across our regions and nations, and I know they expect strong leadership from Unite the union.
"We know that this rabid Tory government is hell-bent on making worker's pay for this health crisis - just like the made worker's pay for the bankers crisis - I am determined to stop that from happening."
Ms Graham, said to be a rising star in the union, has led Unite's campaign for rights for Amazon workers and is bidding to become the union's first woman general secretary.
In what could turn out to be a significant boost for Mr Turner, he narrowly beat Mr Beckett, a solicitor who is the union's in-house lawyer, for the endorsement of the United Left faction of Unite.
But the split on the left could help Mr Coyne, who polled 41.5% to Mr McCluskey's 45.4% when he challenged him four years ago, before being sacked from his union post three months later.