'On Trident Labour in London pulls strings whether its Scottish branch likes it or not'

The decision in question concerns the Royal Navy's fleet of four Vanguard-class submarines, based at Faslane. Crucial to the UK's nuclear posture - they carry the UK's nuclear-armed Trident missiles - these boats are scheduled for retirement by around 2028. A replacement fleet must soon be commissioned if current operations are to continue seamlessly.

Vanguard replacement has been a divisive issue at Westminster. The three main UK parties support Trident but they are at odds over whether the UK can afford to replace all four existing submarines. Disagreement has been so pronounced that the replacement decision has been put off until after this May's election.

It will thus fall to the next UK Government to take the 'Main Gate' investment decision. It will be controversial, taken against the backdrop of claims that the Trident billions would be better invested in health, education and poverty alleviation.

This won't deter the Conservatives if they find themselves in government after the election. If he is Prime Minister, David Cameron is almost certain to announce a like-for-like replacement of all four Vanguard submarines, at a cost of around £20 billion.

If Labour emerge as the largest party in May, things are less certain. Despite its official stance, Trident-scepticism is rife within the party; a recent survey suggested that 75 per cent of prospective Labour parliamentary candidates oppose Trident.

This clearly gives Miliband the political latitude to announce Labour's commitment to ending Trident altogether. Indeed, there has never been a more permissive time for a modern Labour leader to do so: Miliband wants to put clear blue water between himself and the Tories on key policy issues; he needs to address accusations that he is weak; there is a pressing need to channel more defence money into conventional military capacity; it is also necessary to address Labour's current Scottish slump. Ending the Trident programme would go some way towards achieving all of these things.

However, Miliband is unlikely to be so bold. His recent assertion that the UK should have "the least-cost nuclear deterrent we can have" suggests that he favours a downgraded Trident system, operating with three, or even two, submarines.

When viewed from Scotland, it is intriguing to speculate on how a Trident renewal decision from Mr Miliband might impact Scottish Labour's fortunes at a time when it is haemorrhaging support to the pro-independence, anti-Trident parties.

It is a decision which could well cement the current political standings in Scotland. It would certainly play into the hands of a buoyant SNP which would take every opportunity to remind Scots that Labour had spurned the chance to "do the right thing", deciding instead to keep Scotland chained to a costly nuclear future.

Scottish Labour would likely face a stiff challenge in deflecting such criticisms. It is known that Jim Murphy's pro-Trident stance isn't shared by many Scottish Labour MPs and MSPs. This would make it extremely difficult to corral a convincing Scottish Labour line on the issue. If dissent were to manifest itself with any vigour, the circling critics would likely seize upon a recent, damaging observation: that on Trident, as on everything else, it is Labour in London who pulls the strings whether its Scottish branch office likes it or not.

Labour is currently working hard to ensure that it will be taking the big political decisions after May. But success in this endeavour will hand Mr Miliband responsibility for a decision which won't go down well with many Scots, and which may do little to help drag Scottish Labour out of its current malaise.

Dr John MacDonald is Director of the Scottish Global Forum