The race to replace Len McCluskey as the head of the Unite union has formally begun with both left-wing and centrist candidates declaring it’s time for “change”.

Britain’s second largest union is expected to have a new general secretary in post by the end of August after its executive council triggered the election on Wednesday.

Left-backed Steve Turner, seen as the favourite, and centrist Gerard Coyne, an arch critic of McCluskey’s 11-year reign, both swiftly declared themselves as contenders.

Howard Beckett, a vociferous opponent of Keir Starmer on Labour’s ruling National Executive Committee, and national organiser Sharon Graham are also set to stand.

The declarations came as a row blew up over a new threshold to get on the ballot paper after Unite increased the previous minimum requirement for nominations.

The number of local union branches needed by a candidate has more than tripled from 50 in 2017 to 174 today.

A union spokesperson said: “After a full discussion by our executive council, it has been agreed that Unite’s general secretary election is now underway.

“A timetable will be circulated once agreed with the independent scrutineer. The election will be concluded towards the end of August.”

Coyne, who has also hit out at McCluskey’s spending of £98m of members’ money on a Birmingham hotel and conference centre, attacked “the most draconian entry requirements of any election in trade union history”.

The former regional secretary, who came within 5,000 votes of beating McCluskey four years ago, said the tactic would backfire as “reps and members will be repulsed by their attack on the democracy of the union and natural justice”.

Coyne pointed out that of the four expected candidates he would be the only one who was not a senior serving official.

“Unite members and reps are crying out for change, not more of the same. Unite has spent nearly £100m building a hotel and conference centre in Birmingham. That’s a clear example of the wrong priorities and the wrong approach,” he said.

“I will use the union’s resources and energy to empower our workplace reps and support our members through the troubled times to come – not to play political games or waste their hard-earned money.”

But Turner also signalled a break with the McCluskey era, focusing more on workplace rights and member services rather than internal Labour Party politics.

The former bus driver and shop steward, who last year won the crucial endorsement of the United Left grouping, is backed by the Communist Party.

He promised a “Charter for Change” with a focus on better services for union members, including a new Unite Assist smartphone app and extended freephone helpline to provide 24/7 support.

“For 39 years I’ve walked in the shoes of our members; listening, learning and leading – from shop steward and officer to assistant general secretary. I’ve been unemployed and know what it’s like to lose your job, struggling to pay bills and put food on the table,” he said.

“It’s my life experiences that drive me, that motivate me to be the best I can be, from helping secure the furlough scheme, to leading the charge in the fight for the jobs of today and the livelihoods of tomorrow.

“We will not allow the pandemic to be exploited by opportunistic bosses, or workers’ wages pinched to boost the riches of profitable companies and line the pockets of multimillionaires.”

McCluskey is arguably the best known trade union leader in the country, with Unite having played a key role in the elections of Ed Miliband and Jeremy Corbyn as Labour leaders.

He has proved a strident critic of Keir Starmer, warning he would end up in “the dustbin of history” if he continued to “marginalise” the left of the Labour movement.

But Unite’s membership has dropped in recent years, making Unison the biggest union in the UK.

Beckett, who has the backing of former Labour general secretary Jenny Formby, is launching his campaign on Sunday on Zoom.

“This is a critical time for workers across our regions and nations, and I know they expect strong leadership from Unite the union,” he said.

“We know that this rabid Tory government is hell-bent on making worker’s pay for this health crisis – just like the made worker’s pay for the bankers crisis – I am determined to stop that from happening.”

National organising officer Sharon Graham said that her role in leading efforts to unionise Amazon proved what kind of general secretary she would be.

“Things cannot return to business as usual post-Covid. If Unite is to face the massive challenge of fighting for jobs, fighting for wages and defending conditions it must be built at the workplace as never before. New times demand new ways to defend workers,” she said.