CONSTRUCTIVE ambiguity plays a crucial role when it comes to nuclear weapons and how the political parties approach the subject of Britain’s own ultimate deterrent.

As Labour leader Jeremy Corbyn, a vice-chairman of CND, prepares to venture to Scotland this week there is a good chance the issue of Trident renewal will be writ large on the campaign trail.

With the prospect of a post-election “arrangement” being raised between a minority Labour government and an enlarged SNP contingent at Westminster, the Nationalists are hopeful that scrapping Trident will be placed high on what Ian Blackford described at the weekend as the party’s “wish-list”.

But one former Labour minister questioned whether the totemic issue of removing nuclear weapons from Scottish soil would even feature in any political negotiations should the SNP hold the balance of power come December 13.

“The Nationalists are interested in one thing and one thing only: an independence referendum; everything else falls into the background,” he said.

The SNP has for years committed itself to removing what it regards as an “immoral and expensive” arsenal of weapons of mass destruction.

The successor generation of submarines, dubbed Dreadnought, has been estimated to cost £179 billion over its lifetime. By the time it is operational in the late 2020s this could have swelled to £200bn given the nature of inflated prices.

With financial pressures building elsewhere, not least in the social care sector because of increased longevity, the debate about the necessity to spend so much money on nuclear weapons is only set to intensify.

On Sunday, General Sir Nicholas Carter, Chief of the Defence Staff, asked: "Who can tell what the world will look like in 2035?" and described Trident as an “insurance policy”.

But it has to be said he did not appear to express the view with any great conviction.

If Nicola Sturgeon holds the balance of power in five weeks’ time, she might feel she will be pushing at an open door given Mr Corbyn’s personal opposition to Trident and Scottish Labour’s own hostility to it. Of course, the UK Labour Party remains supportive of retention but with a new crop of MPs, how long will that last?

Yet the First Minister was keen to stress removing Britain’s nuclear fleet would happen as quickly as safety would allow. Which in practice could mean not for several years.

In 2012, MPs accepted Trident could be "disarmed within days and removed within months" but pointed out relocating Britain's nuclear deterrent would cost several billions of pounds and would take many years to replicate the safety facilities currently in place in Scotland.

Last year, Stuart Crawford, a former SNP adviser, pointed out that the lack of an alternative site meant Trident submarines based at Faslane and the warheads stored at nearby Coulport would take up to 25 years to remove.

Of course, a fundamental point about Britain’s ultimate deterrent is that for it to be effective, an enemy has to believe we could use it. It seems clear to many Mr Corbyn would never be prepared to push the button.

Emily Thornberry, the Shadow Foreign Secretary, was placed on the back foot yesterday when she was challenged on the issue of Trident.

"Jeremy would do anything to protect our country and whether or not we use nuclear weapons is not something we should be talking about in advance," she declared.

Ms Thornberry insisted she did not believe using Britain’s deterrent would be a "decision made by one individual" but “collectively”.

The Conservatives jumped on this. Johnny Mercer, the Defence Minister said: “If Jeremy Corbyn is unable to make crucial decisions to keep our country safe, he is not fit to be Prime Minister.

“That the Shadow Foreign Secretary is openly speculating her leader could be overruled by a committee on a matter as fundamental as using our nuclear deterrent shows just how weak Jeremy Corbyn really is.”

In three weeks’ time the issue of Trident and defence more generally will be raised as heads of government converge in London for a meeting of Nato on December 3/4.

Seven years ago, the SNP engaged in a mighty tussle with itself over whether an independent Scotland should join the international organisation given the western alliance is based on nuclear deterrence.

The leadership made clear that an independent Scotland’s membership would be founded on removing the current nuclear arsenal and that members would only take part in “UN-sanctioned operations”.

But it is noticeable how little the Nationalist hierarchy talks about Nato; it was absent from the defence sections in the SNP's 2015 and 2017 manifestos. The key reference was about working with “international partners”.

Of course, when Donald Trump turns up for next month’s Nato summit the main subject will not be Britain’s nuclear deterrent, Russian aggression or the crisis in the Middle East but Donald Trump.

The chances of the US President lumbering into Britain’s election fray cannot be overstated.

Given the deeply divisive nature of Mr Trump, any presidential outburst will be to Mr Johnson’s advantage or disadvantage, depending on your view. But, it will, of course, make waves.