Unite the Union, Labour’s largest affiliate, has endorsed Rebecca Long-Bailey and Richard Burgon to be leader and deputy leader of the Labour Party. It means that thanks to Unite’s size and the support of smaller, pro-Corbyn trade unions such as the Bakers’ Union, Long-Bailey and Burgon are guaranteed to make the ballot.

It’s a reminder of two things: that whatever happens in April, however big the next Labour leader’s victory, the post remains one with large but not unlimited power. Neither Keir Starmer, the frontrunner, nor Long-Bailey, nor Lisa Nandy, will have secured the support of all of the biggest trade unions within Labour. That will create opportunities if they are skilled at the art of party management – but it also places limits on their power.

But it’s also a reminder that the power of trades union general secretaries is limited, too. Let’s be candid here: the next deputy leader of the Labour Party is going to be Angela Rayner. You know that, I know that, the other candidates for deputy leader know it and Len McCluskey does too. And the next leader of the Labour Party is highly unlikely to be called “Rebecca Long-Bailey”, either because, as all the signs thus far indicate, Keir Starmer wins or Lisa Nandy pulls off a surprise. You know that, I know that, and Len McCluskey does too.

So why not make Unite’s life a little easier, and futureproof their nomination by backing Angela Rayner? The answer is: because United Left, the organised left grouping within Unite, which won a majority of seats on the union's executive council, didn’t want to, and it is they, not McCluskey, who ultimately decide how the endorsement goes.

That isn’t to lowball McCluskey’s influence within Labour: whatever happens, Unite will have three representatives on Labour’s ruling national executive – as well as holding two of the reserved places for trades unions, Diana Holland, their assistant general secretary, is the NEC’s treasurer. But it’s a reminder not only that the next Labour leader’s power will be limited – but so too are the powers of the trades union general secretaries.