Trident nuclear deterrent: 'The ultimate security guarantee' or 'redundant and economically untenable'?

David Cameron's visit to Scotland today will see the Prime Minister reaffirm his commitment to Britain's Trident deterrent system.

The sea-based nuclear weapons system is needed now more than ever, according to Mr Cameron - but critics have questioned why the UK needs a nuclear deterrent at all.

THE CASE FOR TRIDENT:

The Prime Minister has said it would be "foolish" to leave the country defenceless at a time when the "highly unpredictable and aggressive" regime in North Korea was developing ballistic missiles which could eventually threaten Europe.

Trident - a three-part system of submarines, missiles and warheads, was acquired by the Tory government in the early 1980s and replaced the Polaris missile system, which Britain had had since the 1960s.

Its components still have several years of use left, but cannot last forever and would be 'past their sell-by date' in the early 2020s.

Mr Cameron argued today that we need a nuclear deterrent 'as much today as we did when a previous British government embarked on it over six decades ago'.

The Prime Minister wrote in the Daily Telegraph: "Of course, the world has changed dramatically. The Soviet Union no longer exists. But the nuclear threat has not gone away.

"In terms of uncertainty and potential risk it has, if anything, increased."

Mr Cameron said Iran was continuing to defy the will of the international community over its nuclear programme while North Korea may already be building a nuclear arsenal.

"The highly unpredictable and aggressive regime in North Korea recently conducted its third nuclear test and could already have enough fissile material to produce more than a dozen nuclear weapons," he said.

Related: North Korea Army: 'War could break out today'

"Last year North Korea unveiled a long-range ballistic missile which it claims can reach the whole of the United States. If this became a reality it would also affect the whole of Europe, including the UK."

Supporters of Trident also argue that the UK's global influence would be diminished if it reduced or abandoned its nuclear deterrent programme.

It is thought that up to 15,000 jobs would also be lost in the nuclear defence industry if the Prime Minister scaled back Trident.

WHY DO PEOPLE OPPOSE TRIDENT?

Trident divides opinion in the political world - the Tories made replacing it one of their manifesto pledges in 2010, but Lib Dem MPs oppose a 'like-for-like' replacement and instead believe 'alternative nuclear deterrent' should be considered.

Liberal Democrat MP Sir Malcolm Bruce said he was concerned about funds being diverted away from conventional military equipment to pay for Trident.

"We do accept the case for a nuclear deterrent and we are not in favour of unilateral disarmament," he told Sky News.

"We are saying we shouldn't replace Trident on a like-for-like basis but we are looking at alternative nuclear deterrents once Trident has passed its sell-by date.

"But we also recognise that the cost of a nuclear deterrent is extremely high and I know there are many people inside the Ministry Of Defence and the Armed Forces who desperately want to ensure that we have the latest and most up-to-date conventional equipment and would be extremely concerned if that was prejudiced by a very heavy commitment to a budget for replacement of a nuclear deterrent which by definition is not used, as opposed to weaponry which they need."

Campaigners, meanwhile, describe Trident as 'strategically redundant, economically untenable', and an 'indiscriminate weapons system'.

They argue that Britain does not need a nuclear deterrent anymore, now that the Cold War threat from the Soviet Union is over.

The Campaign for Nuclear Disarmament told Yahoo! today: "Opinion polls have consistently shown that a majority of the British people are against replacing Trident.

"Just this week, thousands turned up to protest with us at Britain's nuclear bomb factory.

"It's precisely because of countries like the UK spending billions on new nuclear arsenals that other countries want to get them. It's a vicious circle which we could end by not replacing Trident.

"Cameron calls it a deterrent, but it does nothing to deter the genuine security threats we face like international terrorism."

UK conflict and poverty charity Medact also voiced their concerns, adding: "It is ludicrous to suggest that Trident could protect us from a nuclear attack from North Korea or anywhere else. 

"Britain's nuclear weapons did not stop Argentina from invading the Falkland Islands and North Korea is threatening to attack the US, which has more nuclear weapons than any country in the world."

WHAT HAPPENS NEXT?

As it stands, Britain is likely to renew the Trident system at some point in the next decade.

In 2007 a large majority of MPs backed plans to renew the deterrent, with early estimates putting the cost between £15bn and £20bn.

The Ministry of Defence would foot the bill, despite already having its budget slashed as part of the Coalition's austerity measures.

The government insist the programme will be scrutinised to ensure 'value for money', and it is thought that £3bn worth of savings have already been made.

Trident's renewal is still in its early stages though - 'conceptual work' is currently ongoing in redesigning Trident's components.

The final green light on renewing Trident will not be given until 2016 - after the anticipated date of the next general election.

The submarines alone could take up to 17 years to develop, and the first ones would not be delivered until at least 2028.