A senior figure and ally of Jeremy Corbyn has called for the trade union link with the Labour Party to end.

Christine Shawcroft, a long-time Labour activist, Momentum official and NEC member, wrote on her Facebook page: "It's time to support disaffiliation of the unions from the Labour Party. The party belongs to us, the members."

Excoriating the trade unions, she claimed that union leadership "stick it to the rank-and-file members time after time".

The message has laid bare the tensions which exist at the top of the Labour Party.

You might think this is not so new - that Labour has been in the grip of a civil war ever since Jeremy Corbyn first stood for leader.


But this is not a fight between factions of the Labour Party, it is left versus left and recently, at least, that really is new.

All of this has come to a head over a job almost no one has heard of.

A few weeks ago Iain McNicol, the general secretary of the Labour Party, resigned.

The role is basically an organisational one, the person responsible for running the party on a day-to-day basis and getting it ready for elections.

It is an important job, but Mr McNicol was not favoured by the leadership of the party.

Image: Iain McNicol's resignation has sparked a battle over his successor

He was not on the left nor a Corbyn ally - and many in the leadership will have been pleased he decided to chuck in the towel.

Up to now, most of the unions and especially the largest, Unite, had (on the surface at least) been working hand in glove with Momentum and the wider left of the party.

That has now changed. Unite put up a candidate for the general secretary job, Jennie Formby, very quickly. It was assumed she would be the left's candidate.

Then to the surprise of many, Jon Lansman, ally of Jeremy Corbyn and head of the grassroots group Momentum, decided to throw his hat into the ring.

He said that he was applying "to open up the contest and ensure we have a wide range of candidates" - saying also (in a not so thinly veiled jab at the way unions do business) he wanted to see an end to "the old machine politics".

The truth is this tension has been bubbling away for a while. As one source told me today: "It has just been masked by success."

Perhaps with so many enemies slain and no common Blairite enemy to fight, there is less to bind the left and the unions together.

Image: Jon Lansman surprisingly threw his hat into the ring

But a clash was always coming, for two reasons.

First, much of Momentum and the left see it as their mission to "democratise" the Labour Party and shift power in favour of its members.

The unions, naturally, are not so in favour. They see themselves as the backbone of the Labour Party and as its primary financial benefactors protect their outsized influence in the party's structures with zeal.

As one source told me: "Union general secretaries have a constituency of members they have to deliver for and most members will stand some grandstanding as long as they get the basics right. If the party goes too far in shifting power they have difficulty justifying why they fund it."

Secondly, there are genuine differences in views about policy.

Much of Momentum sees it as the role of Labour to transform capitalism as we know it.

The trade unions' aims have always been narrower. As one source said: "Look, the unions when it comes down to it care about union rights, decent pay and working conditions. They don't have much truck with endless debates on dismantling the economic system."

Trident probably demonstrates this split better than any other issue.

On the one hand, I doubt there are many members of Momentum who would back retaining the UK's nuclear weapons. They see it as morally and politically indefensible.

But union officials - whatever their private views - know they have members who work at Faslane, making the submarines and in the nuclear industry more widely.

It’s the same with Heathrow or fracking. Give a trade unionist the choice between jobs and radicalism, and they’ll choose jobs every single time.

That is, after all, what they are for: they are duty bound to protect their members' interests, exactly the sort of bread and butter "machine politics" Mr Lansman dislikes.

Moreover in the long sweep of Labour history, trade union and left wing tension is not unusual.

For much of the period from the general strike to the 1970s, they were generally seen as being on the right of the party.

In the 1980s, many on the left blamed the union general secretaries for siding with Denis Healey against Tony Benn in the 1981 deputy leadership contest. Intriguingly some have whispered to me that this is a grudge still felt by Lansman and others around him.

Why does all of this matter?

Well, perhaps it shows that the Labour left is not quite as invulnerable as everyone thought.

The Unite and Momentum alliance has proved a potent one. It has helped elect Jeremy Corbyn twice.

If they drift apart, or worse come to blows, it may lay the groundwork for someone from another wing of the party to make new alliances and seize control in future.

We are a long, long way off that but these are the first glimmers that it may be possible.

Jeremy Corbyn - who has backed the Unite candidate in the general secretary contest - will have to navigate these new splits with skill.

As long-time Labour watcher Kevin Maguire wrote recently: "The seismic disputes in the coming years for the heart and soul of Labour may prove to be not between a Jeremy Corbyn cult and its enemies, or wider left versus right clashes but the collective industrial movement up against individual political activists.”

Perhaps the great Ernest Bevin, a union man himself, got it right when he said: “The most conservative man in the world is the British trade unionist when you want to change him."

The Labour left may - slowly, but surely - be about to realise just how wise these words were.