ONE of the most difficult and hotly-contested issues in the run-up to the September 18 vote is the future of the UK's nuclear deterrent on the Clyde.

For decades now Trident has already assumed a contentious place in Scottish public and political life.

But now the ageing submarine system is at the centre of a debate that takes in not only defence, jobs, international responsibilities and Scotland's vision of itself in the globe, but also complex and expensive technical questions.

Alongside North Sea oil, the currency and the separation of national debt, what will happen to the nuclear deterrent is certain to be one of the largest, and potentially most challenging, issues should the Scottish and UK Governments find themselves sitting around the negotiating table discussing independence.

Q What does the SNP say?

A The Scottish Government's White paper on independence says that ministers will aim to expel Trident from Scotland by 2020.

The SNP says it wants to get rid of nuclear weapons from Scottish waters as soon as it is "safe" to do so.

Q Does the SNP want to go further?

Yes. The SNP has also suggested that an independent Scotland could include in its newly-created constitution a ban on nuclear weapons.

What does the pro-Union side say?

Many pro-Union campaigners point to the thousands of jobs linked to Trident remaining on the Clyde. Others argue that the nuclear deterrent has helped keep the peace internationally in recent decades and should be recognised as an important part of the UK's defences.

What are the technical issues?

One problem with Trident leaving Scotland in the event of a Yes vote is that there is no obvious place for it to go.

In the longer term, a new home could be built in England, Wales or Northern Ireland. But that could cost tens of billions of pounds and run into years of delays triggered by local opposition. Some Conservative MPs believe that Trident could move be moved, in the short to medium term, to an ally's territory such as the United States or France.

The Ministry of Defence (MoD) has already warned the price of any such move would be "enormous", while Lord West, the former First Sea Lord, has even suggested it could trigger unilateral disarmament for the remainder of the UK.

What happens in the event of a No Vote?

Although it is likely to stay in Scotland following a No vote, in what shape remains to be seen.

The deterrent needs to be replaced, and what shape that will take is the subject of fierce political debate.

The Conservatives and Labour back a like-for-like replacement, which experts predict could cost more than £20 billion.

The Liberal Democrats have argued heavily for a scaled-back version which could see the number of submarines reduced from four to three or even two.

The party argues this would represent both a stepping down of the UK's nuclear weapons capability and would produce savings running into billions.

Where does public opinion stand?

Divided. An opinion poll earlier this summer found that more Scots believe that in the event of independence Trident should stay in Scotland than want the deterrent to leave.

The British Social Attitudes survey found 41 per cent wanted to keep Trident, while 37 per cent wanted it removed.

The research also found 46 per cent of Scots opposed the principle of Britain having nuclear weapons, with 37 per cent of respondents saying they are in favour of retaining the deterrent.