Q&A: Trident replacement

Gordon Brown is set to tell the United Nations that he is willing to cut the UK's fleet of Trident missile-carrying submarines from four to three.

But Number 10 has also insisted that keeping some nuclear weapons in the UK is "non-negotiable".

Existing Trident submarines will end their working lives in the 2020s.

What is Trident?

A sea-based nuclear weapons system. There are three parts to Trident - submarines, missiles and warheads, and although each component has years of use left, they cannot last indefinitely. The current generation would begin to end their working lives some time in the 2020s. The government says work on a replacement cannot be delayed because the submarines alone could take up to 17 years to develop.

What is the case for UK nuclear weapons?

While he was prime minister, Tony Blair said it would be "unwise and dangerous" for the UK to get rid of its weapons in such an uncertain world. Today's ministers say that while they are committed to long-term global nuclear disarmament, at present maintaining a missile system is essential - particularly when countries like North Korea look set on acquiring their own. Foreign Secretary David Miliband has said the world must not end up in a situation where "responsible" states get rid of their weapons, but "other powers" hold onto them.

There is also an argument that without a Trident replacement the UK's nuclear industry could be severely damaged. Some estimates say that up to 15,000 jobs could be lost - as well as considerable expertise - if a new batch of submarines is not commissioned.

How much will a replacement cost?

The government has put the bill at £15bn to £20bn, but campaign group Greenpeace claims it will run to at least £34bn once extra costs like VAT are factored in.

Officials have also warned that any decision by Mr Brown to reduce the number of Trident submarines from four to three would not result in a 25% cut in costs.

What do the other parties say?

The Conservatives say they support Trident, but that decisions on its future will form part of an overall review of the defence budget which they plan immediately if they win the next election. Earlier this year, Shadow Defence Secretary Liam Fox suggested that a fleet of three submarines, not four, would provide better value for the taxpayer.

TRIDENT MISSILE SYSTEM Missile length: 44ft (13m) Weight: 130,000lb (58,500kg) Diameter: 74 inches (1.9m) Range: More than 4,600 miles (7,400km) Power plant: Three stage solid propellant rocket Cost: £16.8m ($29.1m) per missile Source: Federation of American Scientists How Trident works

The Lib Dems, meanwhile, say they do not want a like-for-like replacement of Trident. Nick Clegg claims costs could run to £100bn for a system that no longer meets Britain's defence needs.

As well as opposition voices, there are many MPs within the Labour party who are against replacement. So much so that when Mr Blair put the decision to a Commons vote in 2007 he had to rely on Tory support to get it through.

What are the arguments against a replacement?

The main one is that the old Cold War threat from the Soviet Union no longer exists and therefore the need for a nuclear weapon no longer exists. It is also said that nuclear weapons are useless in that they could never be used and would not combat the new threats from international terrorism.

The issue of cost is also raised and is especially salient at a time when spending cuts - and possible threats to vital public services - are at the top of the political agenda.

Finally, there is the question of Britain's obligations under the Nuclear Non-Proliferation Treaty and the point made by the Campaign for Nuclear Disarmament that if some states renew their arms it encourages proliferation elsewhere.

What about Trident's legality?

Some argue that under the Nuclear Non-Proliferation Treaty, Britain should not be re-arming. Critics also argue that using Trident would break international law since such a weapon would not be able to distinguish between combatants and civilians.

What is the government's response?

It argues that the treaty does not commit member states to total disarmament, but to negotiations on effective measures and that it has fulfilled this pledge. It also says it has cut its nuclear weapons explosive capacity by 70% since the end of the Cold War, given up bombs carried by aircraft and reduced the operational readiness of its Trident submarines. Only one submarine is on patrol at any one time, it needs several days notice to fire, its warheads have been reduced to 48 and are no longer pre-targeted. The government argues that its nuclear weapons are designed as a deterrent and would only be used as a last resort in self-defence.

Trident submarines are based at Faslane on the River Clyde

Would a new warhead have to be tested?

Britain signed the Comprehensive Nuclear Test Ban Treaty in 1996 and is observing a moratorium on tests though the treaty has not come into force yet. So the testing of new warheads by explosion is in effect banned.

Is Trident independent?

Both Mr Blair and Mr Brown have been at pains to stress Trident's independence, saying its firing does not require the permission, the satellites or the codes of any other country (i.e. the United States). However, critics argue that Britain is technically so dependent on the US that in effect Trident is not an independent system. For example, the British Trident missiles are serviced at a port in the state of Georgia and warhead components are also made in the US.