Len McCluskey, head of Britain’s biggest union Unite which is also the Labour Party’s biggest donor, has resigned from his post and is standing for re-election.

He last did this in 2013, three years into his first five-year term, to avoid embarrassing then Labour leader Ed Miliband with a major union election at the same time as the 2015 general election. On that occasion, his only rival was left-winger Jerry Hicks, who stood on a militant platform of rank-and-file union democracy.

Hicks came second with 80,000 votes (36 per cent), having previously received 22 per cent to McCluskey’s 42 per cent in 2010, ahead of right-wing candidates Les Bayliss and Gail Cartmail.

This time, McCluskey’s main opposition will come from the right, angered by his support for Jeremy Corbyn in this summer’s Labour leadership election. West Midlands regional secretary Gerard Coyne will be their sole candidate, receiving support from many of the right-wing Labour MPs who tried to force Corbyn to resign in July.

This election will therefore not just be about the leadership of one particular union, but about the future of the labour movement as a whole. Whatever our criticisms of McCluskey’s record – including his failure to defend jobs at Grangemouth refinery in November 2013 – socialists in the party and in Unite should support McCluskey against the right on this occasion.

The campaign will run from March 2017 and end on 28 April next year.