Tony Blair has outlined plans to update and replace the UK's Trident nuclear weapons system.
Why is a replacement being discussed?
There are three parts to the Trident system - submarines, missiles and warheads. Although each component has years of use left, they cannot last indefinitely and would begin to end their working lives sometime in the 2020s. A replacement system would need many years of development, so it is being debated now.
Which option has Tony Blair backed?
Tony Blair says the UK must keep a nuclear weapons system. It will do this by joining a US programme to extend the life of the American-made D5 Trident missiles into the 2040s and by building a new generation of submarines in the UK. A decision on new nuclear warheads is not needed until the next parliament, the white paper says. If they are needed they would be made in Britain.
How much will it cost?
Tony Blair said the new submarines would cost between £15bn and £20bn over 30 years and take up 3% of the entire defence budget each year.
When will a decision be taken?
There will now be three months of consultation on the white paper before MPs are asked to vote on the plans to renew/replace the Trident system.
Do these plans need a vote by parliament?
Legally, no. The Cabinet could decide by itself. However, the prime minister has committed himself to a vote by MPs on the plans in March 2007.
Is Tony Blair likely to win a vote?
There are three months to go before the vote, so things may change. But at the moment it would seem likely that the plans would be backed in the House of Commons, although the prime minister might need to rely on Conservative votes to get it through, given opposition from a number of Labour MPs.
Has Mr Blair given any ground to critics?
Mr Blair says he wants a study to look at whether the nuclear submarine fleet could be cut from four to three. He also says he wants to see the number of UK nuclear warheads cut by 20% to about 160.
What is the case for UK nuclear weapons?
Tony Blair said in an uncertain world with places like North Korea and Iran seeking nuclear weapons, it would be "unwise and dangerous" for the UK to get rid of its weapons. He said "it is not utterly fanciful" to "imagine states sponsoring nuclear terrorism from their soil. We know this global terrorism seeks chemical, biological and nuclear devices". Although the Cold War is over, he said no-one could say whether any new threats would emerge.
What other options are there?
The government ruled out unilaterally scrapping the UK's nuclear weapons. Once that decision was taken it also ruled out using air-based, or land-based nuclear weapons systems. There was also an option - suggested by the Commons defence committee and Lib Dem leader Sir Menzies Campbell - to postpone a decision until later. But the government said it would be "imprudent" to assume that a new submarine could be designed and built within the 14 years. Indeed, the white paper says the design and build process for new submarines will take 17 years.
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
What are the arguments against?
The main one is that the old threat from the Soviet Union no longer exists and therefore the need for a nuclear weapon no longer exists. It is 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 - estimated at up to £20bn for the submarines - is also raised. So is the question of Britain's obligations under the Nuclear Non-Proliferation Treaty (see below).
What about its legality?
Some argue that under the Nuclear Non-Proliferation Treaty, Britain should not be re-arming but moving towards total nuclear disarmament. Article VI of the treaty says: "Each of the Parties to the Treaty undertakes to pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament, and on a treaty on general and complete disarmament under strict and effective international control." 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 has cut its nuclear weapons explosive capacity by 70% since the end of the Cold War. It has given up bombs carried by aircraft and has reduced the operational readiness of its four Trident missile 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 and that therefore they are no different in principle and international law from other systems.
Could a new warhead 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. However, there have been reports in the Sunday Times that Britain has been developing a new warhead which does not need testing by detonation. Called the "Reliable Replacement Warhead", it could be tested by computer instead. The work for this would be done at Aldermaston for which major new investment was announced in July 2005.
Is Trident independent?
Tony Blair was at pains to say that firing Trident does not require the permission, the satellites or the codes of any other country (i.e. the United States) and that therefore it is fully operationally independent. However, critics say that Britain is technically so dependent on the United States that in effect Trident is not an independent system. For example, the British Trident missiles are serviced at a US port in Georgia, the missiles are to have their lives extended by the US and Tony Blair has said the UK will work with the US when the US develops a replacement for the D5 missile in the 2040s. The critics also argue that the British warhead design is based on an American one and that warhead components are also from the United States.