Media playback is unsupported on your device Media caption Len McCluskey warns Labour critics over Unite poll

Who leads the Unite union is significant - it is the country's biggest, and has members in many strategically important industries.

And, in most years, it is also the Labour Party's single biggest source of funds.

The election of Unite's general secretary is something of a proxy battle in the struggle for Labour's soul.

There is a widespread belief among many of the party's MPs that if there is an early election, Jeremy Corbyn will not only lead Labour, he will lead it to defeat, and then probably step aside.

There is a hope that if the election takes place as scheduled in 2020, Mr Corbyn will choose to stand aside sooner if polling remains bleak.

Even his shadow home secretary Diane Abbott has suggested he has a year to turn the poor polls around. Under either scenario, there would be another leadership contest in the next few years. Those on the left would want to see Mr Corbyn replaced by a candidate with similar politics.

Leadership rules

But under Labour's current rules, a Corbyn ally might struggle to get the support of enough MPs to enter the race.

If, however, the threshold for getting on the ballot were to reduce from 15% of MPs to 5% then a left candidate would almost certainly exceed it and the increasingly left-wing grass-roots membership would, most likely, vote for them.

Image copyright PA Image caption Gerard Coyne is concerned about the impact of some of Labour's policies on his members

But in order to change those rules, Labour's ruling national executive (NEC) would have to agree.

The current Unite general secretary Len McCluskey so far has staunchly defended Jeremy Corbyn - and should Mr Corbyn choose to go, the union leader would work hard to ensure Labour's rules were altered to make a left-wing successor more likely.

As well as having its own representatives in the trade union section of the NEC, next month's merger with the construction union UCATT will boost Unite's influence on Labour's ruling body. And although not elected in the trade union section, insiders point out that as a Unite member the party's treasurer - Diana Holland - can also be relied upon to vote with her colleagues on the NEC.

All in all the union can rely on five votes - or one in seven - of NEC members. That doesn't sound much, but the party's ruling body is finely balanced between Corbyn critics and supporters and some recent votes have been decided on very narrow margins.

Mr McCluskey's main challenger Gerard Coyne is, privately, backed by some prominent anti-Corbyn MPs and peers. His pitch is that he would concentrate on defending members' jobs and conditions and spend less time on internal Labour politics.

And if he were elected, anti-Corbyn MPs believe that he would act as a bulwark against further attempts to move the party to the left.

Mr Coyne's supporters are confident that he stands a chance of winning. Three years ago Len McCluskey triumphed on a turnout of just 15%. Mr Coyne's backers believe he can boost participation - and he already has a solid base in the West Midlands and in some of the union's biggest branches.

'Course of history'

Tom Watson, Labour's deputy leader and a West Midlands MP, has been personally if not politically close to Len McCluskey in the past.

He won't officially endorse a candidate but he gave me a far-from-ringing endorsement of his old friend's time in office.

Image copyright PA Image caption Unite is Labour's most consistent financial backer

"It is a golden rule for Labour politicians never to interfere in union elections but (Unite members) have a big choice to make - British workers have had a terrible decade when it comes to pay and conditions and they will want to know the person leading the biggest union is totally focused on their interests.

"I say to Unite members make sure you use your vote - it's a vital election and very often people can win elections like these on a very low turnout.

"The general secretary has a great influence on politics. Len McCluskey is a close personal friend of Jeremy Corbyn. Members can help change the course of history if they decide to vote."

Faced with Gerard Coyne's challenge, Mr McCluskey has been defending himself on his right flank by recognising that the free movement of labour has exercised a downward pressure on wages - though he has also pointed out that reports suggesting he favours restricting migration are "a deliberate lie".

But the entry in to the race of a candidate to the left of Mr McCluskey has unsettled the incumbent's supporters. They are sceptical that grass-roots member Ian Allinson will get the necessary nominations from 50 of the union's branches to compete in next spring's ballot - but if he does, it could cost Len McCluskey support.

'Political football'

Last time round, challenger Jerry Hicks also attacked Mr McCluskey from the left and got more than a third of the vote.

Ian Allinson is portraying Mr McCluskey as an "establishment candidate" and positioning himself as the true friend of Jeremy Corbyn - thus potentially splitting the left vote.

In the statement declaring his candidacy, he said "it is essential this election does not become a battle between Len McCluskey, offering more of the same, and Gerard Coyne, offering to turn the clock back to the bad old days when our union backed New Labour".

Mr McCluskey was dismissive of Mr Allinson when I spoke to him - pointing out that his prospective rival isn't a Labour Party member.

Instead he was more concerned about the motives of those to his right and see the challenge from Gerard Coyne as an attempt to destabilise Jeremy Corbyn.

"There is no doubt the right wing of the Labour Party are going to try to use Unite as a political football and Gerard Coyne is falling in to this trap.

"I have heard him say I spend too much time in the political arena - he knows that's not true. Ninety-five per cent of my time is spent on industrial matters.

"My members won't take kindly to anybody outside the union abusing the democracy of the union by using it as a proxy war."

But whether it ought to be or not, it's almost certain the battle within Unite will reflect the continuing tensions within Labour.