Last night's Labour rebellion over the Conservatives' fiscal charter was smaller than party whips originally feared. Twenty one of Jeremy Corbyn's MPs abstained, rather than voting against the measure. But an issue on which Labour is far more divided is Trident. Only a handful of shadow cabinet members (Diane Abbott, John McDonnell, Jon Trickett, Ian Murray) share Corbyn's unilateralist stance, with a majority, including shadow defence secretary Maria Eagle, opposed.

The "maingate" vote on Trident renewal has long been scheduled for next year (around June) but the Tories are reportedly considering bringing it forward to December to exploit Labour's divisions. Eagle denounced the plan as a "despicable attempt to play politics" when I interviewed her this week.

Now, the anti-Trident SNP is also seeking to cause trouble for Labour. In his speech at the party's conference, Westminster leader Angus Robertson announced that the nationalists would seek to use one of their opposition day debates to trigger a vote on the issue. He said:

Can I just say to Jeremy Corbyn that one U-turn that he can’t consider is ditching his principled career-long opposition to Trident. We will have to decide shortly at Westminster on Trident, and the SNP will resolutely oppose the renewal of weapons of mass destruction and the enormous waste of £100bn pounds. Labour weren’t prepared to have a specific debate on Trident at their conference. I am pleased that the SNP is debating it here at our conference. I’m also happy to help Jeremy Corbyn and ensure that there is an early debate and vote at Westminster aimed at opposing Trident renewal. Hopefully Jeremy will join the SNP in the lobbies as he has often done in the past in opposing Trident, although I am not holding my breath that he can bring his colleagues in the Parliamentary Labour Party with him.

The prospect of an early vote on Trident leaves Corbyn with little time to reach a position. Eagle is about to begin a review of defence policy, including nuclear weapons, for the party. Should Corbyn whip MPs in favour of the SNP motion, he would risk a mass rebellion, including possible shadow cabinet resignations. Alternatively, he could allow a free vote, or simply ignore the motion (which is purely symbolic). But this would hand the SNP a chance to denounce Corbyn as "weak" and reaffirm its status as the only major anti-Trident party. Rather than attacking the Labour leader's policies, the nationalists' focus since his election has been on presenting Labour as too divided to "stand up for Scotland". Few issues provide better opportunities to do so than Trident. It is for this reason, among others, that Corbyn will find it so hard to make progress north of the border.