Jeremy Corbyn’s most powerful union ally, Len McCluskey, has been bitterly attacked by the man seeking to end his six-year reign in charge of Britain’s biggest union, Unite.

In his first newspaper interview since announcing that he will challenge McCluskey in an election, with huge potential repercussions for Corbyn and for Labour, Gerard Coyne accuses the Unite general secretary of meddling too much in Westminster politics, and of neglecting the crucial issues facing working people.

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“I just don’t think that ever again the general secretary should be the puppet master of the leader of the Labour party,” Coyne – Unite’s regional secretary in the West Midlands – told the Observer. “There is an opportunity for change, for a fresh start, for members to get their union back.”

The battle for control of Unite is being seen at Westminster as a proxy war for control of the entire Labour party. Many Labour MPs see Coyne’s challenge as a chance to break the alliance between Corbyn and the leftwing Unite leadership and restore it to the centre ground.

In July, McCluskey accused MPs who plotted to oust Corbyn of “being seduced by sinister forces”. At its annual conference, Unite then called for mandatory reselection of Labour MPs in a move which many saw as the start of a purge of moderates by the hard left.

Labour MPs have since said that Unite activists have been organising in their constituencies in an effort to boost memberships and tip the balance in favour of deselections in future ballots.

Coyne says he, as much as anyone, wants a Labour government and that he fully endorses the commitment in Unite’s rulebook to back the party. But he claims McCluskey has made politics too much of a priority at the expense of fighting everyday battles on behalf of Unite’s 1.4 million members.

Facebook Twitter Pinterest Len McCluskey at the Labour party conference in September. Photograph: Leon Neal/Getty Images

“The last time that I can remember the general secretary making a public appearance on an industrial matter was the last British Airways dispute.

“That was four years ago,” says Coyne. “Every other appearance on the media that I can recall was in relation to the current leader of the Labour party, or in relation to Ed Miliband.”

The 66-year-old McCluskey, a former Liverpool docker who has been general secretary since 2011, threw the super-union’s full support and financial resources behind Corbyn in the leadership elections last year. He has continued to be one of Corbyn’s staunchest and most consistent defenders.

The Unite leader triggered the leadership contest by resigning well before the end of his five-year term in 2018 in what was seen as an attempt to ensure that he stays in charge for the 2020 general election.

Launching his re-election bid last week, McCluskey called for restrictions on immigration. “While we must reject any form of racism, and help refugees fleeing war, we must also listen to the concerns of working people,” he said – a sentiment echoed by Coyne who said the main lesson of the 23 June referendum was that people wanted to restore controls at UK borders.

Coyne also suggests the union under McCluskey has neglected the cause of women in the workplace, and not done enough to promote equal and flexible working. Asked if McCluskey was too old to seek another term, he said: “The simple answer to that is yes.”

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Candidates for general secretary must gather nominations from at least 50 workplaces or branches by 22 February. Voting will take place between late March and mid-April.

McCluskey is a former flatmate of Tom Watson, Labour’s deputy leader, but the pair have fallen out badly since McCluskey criticised New Labour for wielding “power without principles”.

Watson replied: “Trashing our own record is not the way to enhance our brand.”