Len McCluskey will warn Jeremy Corbyn not to force through a change of policy over Trident, according to PoliticsHome's Kevin Schofield.

The leader of Britain's largest trade union will speak out on the subject on Sunday at Unite's Scottish policy conference, potentially putting the Labour leader in a tricky spot over the renewal of the nuclear deterrent.

Trident is an emotional issue for much of the Labour party in parliament and the country but an economic one for Unite and the GMB, two of Britain's largest trade unions and both power-players in the Labour party. Both have large numbers of members in the defence sector and the two unions are in competition for members - neither McCluskey nor Sir Paul Kenny, the outgoing head of the GMB, want to be seen as being anything other than supportive of the deterrent.

For Corbyn, it raises the troubling prospect of putting him at loggerheads with the leaders of Labour's three largest affliated unions. Unite and the GMB both support retaining the deterrent and combined with members of Labour's centre and right flanks to keep Labour's support for Trident at last year's Labour party conference. Unison, the biggest public sector workers' union, does not have a direct stake in Trident renewal but Dave Prentis, the recently re-elected general secretary, is a Corbyn-sceptic who has been vocally critical of the Labour leader in the recent past.

The difficulty for Corbyn is that the leaders of all three unions are privately sceptical at best to his leadership. McCluskey tried and failed to get his executive committee to back Andy Burnham for the party leadership, while Prentis had planned to back Yvette Cooper while bowing to reality. Kenny avoided the fight by keeping the GMB neutral, but it is believed that had he faced the prospect of re-election, he too would have thought it prudent to endorse Corbyn. His successor, Tim Roache, is not believed to be a Corbynite either. Of the affliated unions, only Dave Ward of the CWU and Matt Wrack of the Firefighters' Union are true allies - Ward lent staff and resources to the Corbyn campaign and continues to do so now, while Wrack reaffliated following Corbyn's victory.

But the reality is that if McCluskey, Roache and Prentis combine they are the only part of the Labour movement with the potential power to bring down Corbyn. All are aware that without a change in government, the Trade Union Bill will remain on the statute books. However, the same bill that unnerves the general secretaries makes them weaker than they have been for some time. For Unison in particular, the moves to eliminate "check off payments", where union subscriptions are deducted by the employer at source, will mean a whopping reduction in that trade union's funding. All of the trade unions will be less able to give large sums to Labour - and therefore will be able to wield a smaller fiscal sanction than in times past.

Unite are somethiing of an exception, in that they still have a large amount of political funds in reserves, all of which could, theoretically, be given to the Labour party and could form the basis of a stand-down between Unite and the Labour leadership. But the picture is complicated by McCluskey's own coming re-election battle. McCluskey does not have to seek re-election until 2018 but his mandate from his left flank - including groups like Counterfire, the Socialist Party and others - expires in February 2016. Losing that battle would open McCluskey up to the damaging possibility of a serious challenge to his left. Unite's general secretary is elected by first-past-the-post, so losing votes to a left challenger is more dangerous to McCluskey than it otherwise would be. With one eye on that battle, he is unlikely to open up a sustained attack on the Labour leader while the support of Corbyn-inclined groupings in Unite is very much up for grabs. The bigger risk to Corbyn from McCluskey is if he remains in post at the top of Unite after 2018.