The battle to become the leader of the UK's largest trade union is coming to an end as Unite sends out ballots to more than 1.2 million of its members on Monday (27 March).

Current general secretary Len McCluskey, who was elected to the post in 2011, has faced a bitter campaign against main rival Gerard Coyne.

Coyne, Unite's regional secretary for the West Midlands, has accused McCluskey of being Labour leader Jeremy Corbyn's "puppetmaster".

"People really do believe it's time for a change and a sense that under Len's leadership we've spent far too much time focusing on Westminster politics and not enough time on supporting our members and protecting them through difficult times," he told IBTimes UK.

McCluskey, an ally of Corbyn, gave the left-winger 15 months to turn Labour's low ratings in the national opinion polls around.

The deadline comes after the party's historic defeat in the Copeland by-election in February. It was the first time since 1982 that an opposition party had lost to the government in a by-election.

"I would suggest that the next 15 months or so will give us the answer to that," he told BBC Radio 5 Live on Sunday 26 March. "I think it will give us an answer. In the meantime Unite as we always do – because we're the biggest affiliate – will support the Labour Party."

McCluskey had rowed with Tom Watson after the Labour deputy leader claimed Momentum founder Jon Lansman would "destroy" the party if the pro-Corbyn pressure group affiliated with Unite, a major Labour donor.

Watson and Corbyn later issued a joint "unity" statement after "robust" peace talks.

"The shadow cabinet agreed on the need to strengthen party unity. It recognised the right of groups across the spectrum of Labour's broad church to discuss their views and try to influence the party so long as they operate within the rules," Corbyn and Watson said.

"The leadership represents the whole party and not any one strand within it. No one speaks for the leadership except the leadership themselves and their spokespeople.

"The shadow cabinet agreed our local and mayoral election strategy and what a united Labour Party can and must offer the whole country after seven years of Tory austerity in terms of jobs, housing, education and health and social care.

"We will fight for a Britain where people aren't held back and where everyone in every community can lead a richer life."

The Unite election will conclude on 28 April and McCluskey is expected to win the contest.