Q&A: Trident replacement
Ministers have given the go ahead for the first phase of a process which could lead to the renewal of the UK's fleet of Trident nuclear missile submarines.
The government says it is committed to retaining a minimum nuclear deterrent, saying a like-for-like replacement remains the best option, but tensions remain between the Conservatives and Lib Dems over the issue.What is Trident?
A sea-based nuclear weapons system. 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.
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 of four submarines would begin to end their working lives some time in the 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, "conceptual" work has been going on considering potential designs for replacement submarines, propulsion systems and other key components.
Last October, the government decided to delay the ultimate decision on whether to proceed and how many submarines to order until 2016, after the expected date of the next election.
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.
But Defence Secretary Liam Fox has now approved what is known as the "Initial Gate" phase in which procurement of important items, costing £3bn, will take place.What are the politics involved in renewal?
Prime Minister David Cameron has always maintained that the UK needs to keep its nuclear weapons, calling it as "insurance policy" against attacks. Replacing Trident was a Tory manifesto pledge in the 2010 general election.
But the Lib Dems have always been sceptical about the merits of a like-for-like replacement.
As part of the Conservative-Lib Dem coalition agreement, the government said Trident would be replaced but the programme would be scrutinised to ensure "value for money".
The Lib Dems are continuing to make the case against renewing Trident.
And in an apparent concession, Dr Fox has said the Ministry of Defence will conduct a review to see if cheaper, possibly land-based alternatives to Trident, are feasible.
Labour supports Trident renewal, saying it has been a "cornerstone" of peace and security for nearly 50 years - although a number of MPs on the left of the party have historically opposed it.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.
The Ministry of Defence, which will have to foot the bill for renewal at a time its budget is being cut, said the value for money process had already identified £3bn in potential savings.
Officials have warned any decision to reduce the number of Trident submarines from four to three would not result in a 25% cut in costs although, when in opposition, Dr Fox said having three submarines would provide better value for money for the taxpayer.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 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.
The cost issue is also especially salient at a time of extensive government spending cuts.
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.
Whether other potential alternatives would be much cheaper in practice is also disputed. If Britain chose a cruise missile system, it would probably have to develop its own missile programme, bearing all the research and development costs.
Others have suggested using a land-based delivery system, to avoid the cost of building new submarines. But that has been rejected in the past as too vulnerable to attack and impractical.
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.Would a new warhead have to be tested?
Britain signed the Comprehensive Nuclear Test Ban Treaty in 1996 and is observing a freeze 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?
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.