A guide to Trident and the debate about its replacement

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The Conservatives say replacing Trident in its current form is the best option. Labour has been in favour too, but new leader Jeremy Corbyn opposes nuclear weapons and the party's policy is being reviewed.

What is Trident for?

Since 1969, according to government documents, a British submarine carrying nuclear weapons has always been on patrol, gliding silently beneath the waves, somewhere in the world's oceans.

The logic is to deter a nuclear attack on the UK because, even if the nation's conventional defence capabilities were destroyed, the silent submarine would still be able to launch a catastrophic retaliatory strike on the aggressor, a concept known as mutually assured destruction.

The submarines carry up to 8 Trident missiles, each can be fitted with a number of warheads, which can be directed at a range of different targets.

Each of the four submarines carries a sealed "letter of last resort" in the prime minister's hand, containing instructions to follow if the UK has been devastated by a nuclear strike and the government annihilated.

What is Trident's history?

It was acquired by the Thatcher government in the early 1980s as a replacement for the Polaris missile system which the UK had possessed since the 1960s.

Trident then came into use in the 1990s. There are three parts to Trident - submarines, missiles and warheads. Although each component has years of use left, they cannot last indefinitely. The current generation of four submarines would begin to end their working lives some time in the late 2020s.

Work on a replacement cannot be delayed because the submarines alone could take up to 17 years to develop. Only one submarine is on patrol at any one time and it needs several days' notice to fire.

What is the case for UK nuclear weapons?

Supporters say Trident is indispensable for protecting the UK's security. They argue that new threats, both from rogue states and terrorist groups, could emerge at any time and a minimum nuclear deterrent is needed to help counter them. Despite progress in global counter-proliferation and multilateral disarmament, supporters point out that countries continue to acquire nuclear capability and the UK's global influence would be diminished if it unilaterally gave it up. The nuclear defence industry is also a major employer. Some estimates suggest that up to 15,000 jobs may be lost - as well as considerable expertise - if a new batch of submarines is not commissioned.

Is the renewal going ahead?

As we stand, yes, but the final decisions have not been taken yet. Since 2007, when MPs backed plans to renew Trident by 409 to 61 votes, "conceptual" work has been going on considering potential designs for replacement submarines, propulsion systems and other key components. The "Initial Gate" phase, consisting of £3bn in procurement of important items, has also been approved. But in October 2010, the government decided to delay the ultimate decision on whether to proceed and how many submarines to order until 2016. Given that the Conservatives won the 2015 election and back renewal, it would be a surprise if replacement did not go ahead. The delivery date for the first submarine was also put back to 2028 while the number of operational missiles carried will be cut to eight and the number of warheads to 40.

What are the politics involved?

Conservative leader and Prime Minister David Cameron has always maintained the UK needs to keep its nuclear weapons, calling it as "insurance policy" against attacks. Replacing Trident was a Tory manifesto pledge in the general election.

Labour has supported Trident renewal, saying it has been a "cornerstone" of peace and security for nearly 50 years - but that policy is now in doubt after the election of long-time opponent Jeremy Corbyn as party leader. He says the issue will form part of their defence review, but has also said that even if there were a replacement system, he would never use them as PM.

The SNP, which now has 56 MPs in the House of Commons, opposes Trident renewal. During the election campaign it described Trident as "unusable and indefensible - and the plans to renew it are ludicrous on both defence and financial grounds".

The Lib Dems, who insisted on no final decision being taken while they were in coalition, have always been sceptical about a like-for-like replacement and insisted on a value for money review. They back a "step down the nuclear ladder" with a smaller nuclear weapons system providing a "minimal yet credible" deterrent.

How much will a replacement cost?

The government has put the bill at between £15bn and £20bn, but campaign group Greenpeace claims it will run to at least £34bn once extra costs like VAT are factored in. Jeremy Corbyn quotes a figure of £100bn. The Lib Dems say ordering fewer submarines would save up to £4bn in the long term but Conservatives have rejected this - saying the savings made would be "trivial" in respect of the Ministry of Defence's annual £34bn budget.

Currently, the government is spending around 6% of its annual defence budget on trident, the Ministry of Defence (MoD) has confirmed.

What are the arguments against a replacement?

The cost issue is particularly salient at a time of extensive government spending cuts, set to continue beyond 2015. In terms of defence, the main argument is that the old Cold War threat from the Soviet Union no longer exists and therefore the UK no longer needs nuclear weapons, or does not need a submarine-based system designed for the Cold War era.

Critics say nuclear weapons are useless in that they could never be used and would not combat the new threats from international terrorism. Finally, there is the question of Britain's obligations under the Nuclear Non-Proliferation Treaty. The Campaign for Nuclear Disarmament argues if some states renew their arms it encourages proliferation elsewhere.

What are the alternatives?

Trident's ballistic missiles have a long range, of up to 7,500 miles. One alternative that has been suggested is using cruise missiles based on different submarines. But cruise missiles have a far shorter range, of over 1,000 miles, and are slower and vulnerable to being shot down. The government review concluded this would actually cost more than renewing Trident in its current form, since the UK may have to bear all the research and development costs of its own programme.

Others have suggested using a land-based delivery system, to avoid the cost of building new submarines. That has been rejected in the past as too vulnerable to attack and impractical although the 2013 options review said this could potentially be mitigated by having fewer "silo" sites that were more strategically located.

Some say it would be cheaper to launch missiles from a long-range aircraft. However, the shorter range would again be an issue - and the aircraft could be brought down. The review said "much more work" would be needed on such an idea.

Is Trident independent?

Past prime ministers have always stressed Trident's independence, saying its firing does not require the permission, the satellites or the codes of the US.

However, critics argue 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.

As part of the renewal, common missile compartment systems that could be fitted into both UK and US boats are set to be developed as a means of saving money.