Key Corbyn-ally Len McCluskey is to be challenged for the leadership of Unite the union, Labour’s biggest financial backer.

Unite's West Midlands secretary Gerard Coyne is making the move amid concern about his union’s position on key issues and its involvement in what he called "Westminster power games".

Unite under Mr McCluskey has provided crucial backing for Jeremy Corbyn, but potential Labour policies – including a possible ban on petrol cars revealed by The Independent – have led to concerns over the impact on Unite members’ jobs.

In a Facebook post Mr Coyne said: "I believe our union needs to be changed so we can get on with the only job that matters; protecting and defending our members at work. That’s the job I’ll carry out if I’m elected."

He went on: "We have got to be strong, smart and organised but we can’t be distracted party politics. As a candidate, my priority is making a real difference for you rather than playing Westminster power games.

"It’s time we changed the union to reflect our members’ priorities."

The outcome of the contest, which will run into next spring, will dictate who has control over a huge network of political supporters and campaign infrastructure which under Mr McCluskey have been mobilised for Mr Corbyn.

But after news broke of the potential petrol-car ban, recently mooted in the leader's office but not currently adopted, one union insider told The Independent: "It’s a crazy policy. They’re not thinking."

Mr Corbyn’s anti-Trident stance has also sounded alarm bells with some in the union, given that thousands of Unite’s 1.4m members work in the defence industry.

Len McCluskey says 'sinister forces' are behind Corbyn coup

But the bid from Mr Coyne, who has worked for Unite for more than 20 years, will also be seen as a politically motivated attack by some of the Labour leader's allies.

Mr McCluskey announced at the start of the month that he would stand down from his position in order to run again, allowing him to stay in charge until 2020, when the next general election is due.