On the morning of 9 July, one man precipitated the start of Labour’s leadership contest: Len McCluskey. The refusal of Unite’s general secretary to countenance the resignation of Jeremy Corbyn led Tom Watson, the party’s deputy leader, to cease negotiations. McCluskey responded in his characteristically pugnacious style. “An act of sabotage fraught with peril for the future of the Labour Party” was how he described his former flatmate’s actions.

McCluskey’s central role was fitting. Without his trade union, Corbyn would never have become Labour leader. In the summer of 2013, Unite campaigned for the selection of Karie Murphy, a friend of McCluskey (and now the director of Corbyn’s office), as Labour’s parliamentary candidate in Falkirk. It was after allegations of vote-rigging by Unite that Ed Miliband resolved to reform his party’s relationship with the unions, including their role in leadership elections.

On 1 March 2014, Labour replaced its electoral college – which gave a third of the vote each to MPs and MEPs, party members and affiliated trade unionists – with a “one member, one vote” model. This system enabled Corbyn’s triumph.

The Labour leader’s candidacy was backed by just 15 MPs but he was endorsed by Unite, the party’s largest affiliate and donor. The 1.4 million member union provided office space for the campaign and contributed £175,000. McCluskey is said to have favoured Andy Burnham privately (a claim that Unite denies) but his executive committee voted to back Corbyn.

Although Unite’s general secretary lies on Labour’s far left, he is a comparative moderate in his union. In 2014, United Left, a faction that includes supporters of the Socialist Party, Militant’s successor group, won 43 of the executive’s 63 seats. The low turnout in Unite’s internal elections – 15.2 per cent voted in the 2013 general secretary contest – hands disproportionate influence to its radical wing. McCluskey’s defining political task has been to appease it while maintaining support for Labour.

Born in Liverpool in 1950, McCluskey left school to work on the Merseyside docks, becoming a shop steward for the Transport and General Workers’ Union (TGWU) at the age of 19. “He’s from a humble background. He got to the top through force of personality, intellect and organising skills,” Jon Trickett, the shadow business secretary and a Corbyn ally, told me.

Although he joined Labour in 1970, McCluskey became a supporter of Militant, the Trotskyist entryist group that Neil Kinnock later expelled. “All the people who are mad, bad and dangerous to know are Len’s old pals,” a senior MP told me. McCluskey’s chief of staff is Andrew Murray, a member of the Communist Party of Britain, who has defended North Korea’s Stalinist regime (“Our party has already made its basic position of solidarity with People’s Korea clear,” he declared in 2003).

McCluskey served as the national secretary of the TGWU general workers’ group from 1990 and in 2007 was appointed as the assistant general secretary for industrial strategy of the newly created Unite (founded through a merger of the TGWU and Amicus). It was in this role that he achieved national renown, earning the moniker “Red Len” for his championing of the 2010 British Airways cabin crew strike.

In November that year, he was elected as general secretary. Unite played a decisive role in the Labour leadership contest that year through its endorsement of Ed Miliband. “Thank you,” said Miliband after the result was announced, putting his arm around the co-general secretaries, Derek Simpson and Tony Woodley.

During the Labour leader’s first conference speech, however, McCluskey – who was elected as general secretary soon afterwards – fired a warning shot. “Rubbish!” he cried from the floor, after Miliband condemned “overblown rhetoric about waves of irresponsible strikes”.

As Labour became increasingly dependent on Unite’s funding, McCluskey’s influence grew. After Miliband supported a public-sector pay cap of 1 per cent and further spending cuts, Unite’s general secretary accused him of “being dragged back into the swamp of bond-market orthodoxy”. “Ed barely spoke about fiscal responsibility after that,” a former shadow cabinet minister told me.

In an interview with me for the New Statesman in 2013, McCluskey warned that if Miliband was “seduced” by “the Blairites” (named as Douglas Alexander, Liam Byrne and Jim Murphy), “he’ll be defeated and he’ll be cast into the dustbin of history.” In the subsequent shadow cabinet reshuffle, two of the three were demoted.

With the notable exception of Trident renewal, a project on which thousands of Unite members’ jobs depend, McCluskey’s relations with Corbyn have been far more cordial. Yet his support for the Labour leader has never been unconditional. In January 2016, he stated that Corbyn had “two or three years” to develop an alternative to austerity. The underlying message was clear: Unite’s support is not indefinite.

Labour’s rebels long hoped that after his anticipated 2018 bid for re-election, McCluskey would turn against Corbyn. “He wouldn’t need to keep the left onside then,” one told me. But the political deus ex machina that they sought is now more distant than ever. To defeat Corbyn, Labour MPs will also have to defeat McCluskey.