The Big Question: Should Trident be replaced, and does Britain really need nuclear weapons?

Why are we asking this question now?

Yesterday's meeting of the Cabinet marked the start of a process that will decide whether Britain replaces its independent nuclear deterrent when the current system comes to the end of its life. A White Paper on the options for replacing the Trident nuclear missile system will be published before Christmas, and MPs will have the chance to debate and vote on the Government's favoured option after a "period of debate" in the new year.

Tony Blair argues a decision is needed now because planning Britain's nuclear defence policy is a long-term business. The submarine-launched Trident nuclear missile system that has provided Britain's nuclear arsenal since 1994 is expected to start coming to the end of its life in 2024. A replacement could take 17 years to build, meaning that a decision would be needed now for a new system to be ready.

The hugely controversial nature of the debate about nuclear weapons also means it is politically desirable for Mr Blair to push through a potentially divisive decision before he leaves Downing Street to clear the decks for his successor.

Why do we have nuclear weapons?

Britain's first nuclear test in October 1952 was spurred on by fears of the Soviet Union's conventional and nuclear firepower in Europe and the rising tensions at the start of the Cold War. The systems were justified as deterrents, which would prevent nuclear war by guaranteeing the destruction of any power that launched a nuclear strike against the United Kingdom. However, after the end of the Cold War in the late 1980s and the collapse of the Soviet Union, the then Conservative scaled down Britain's nuclear arsenal, scrapping so-called "sub-strategic" weapons and leaving Trident as Britain's only nuclear delivery system.

The Labour government scaled Britain's nuclear arsenal back still further in 1998, cutting the number of warheads below 200, and reducing the number of warheads carried on each submarine patrol. The 1998 strategic defence review acknowledged that threats had reduced, but insisted that "nevertheless, while large nuclear arsenals and risks of proliferation remain, our minimum deterrent remains a necessary element of our security".

How big is Britain's nuclear arsenal?

The UK holds up to 200 active 80-100 kiloton nuclear warheads, each representing about eight times the explosive power of the Hiroshima bomb. The warheads, built and maintained at the Aldermaston atomic weapons establishment, are fitted to American-built Trident missiles. They can be fired from any of Britain's four ballistic missile submarines, which roam the seas, hidden and ready to strike.

Britain's arsenal is smaller than France's estimated 348 warheads and are dwarfed by the 5,736 active warheads held by the US and the 5,830 thought to be held by Russia.

What would it cost to replace Trident?

Options include extending the life of the current system, buying a new submarine-launched weapon or using an air-launched system instead. Trident cost £14.9bn to buy at today's prices and costs between £1.9bn and £2.2bn a year to run. The cost of an alternative would depend on the type of nuclear weapon the Government might choose to buy. Estimates range between £15 bn and £25, with some claims suggesting a Trident replacement could cost £75 bn to buy and maintain over its lifetime. The latest indications are that the Government will opt for a new submarine-based system, likely to cost about £30bn.

Do nuclear weapons reduce the threats facing Britain today?

Some analysts maintain that the collapse of the Soviet Union has removed the threat of a superpower with its nuclear arsenal trained on the West. They argue that Russia and its former Soviet republics are now allied to the West and their weapons no longer pose a real threat to British security. The late Robin Cook argued last year that as no other nation has replaced the Soviet nuclear threat, there is no longer a justification for Britain to hold a deterrent.

However, backers of Britain's continuing nuclear arsenal point to the large stocks of weapons still held around the world, and point out that nuclear powers such as China could be a source of future instability, while Russia could become destabilised and posed a renewed threat to the West. They also point to the possibility of new nuclear threats from states such as North Korea or Iran. There is also a potential threat from terrorists obtaining nuclear material, but critics claim a British nuclear weapon would be of little use as a deterrent against such a threat.

Are there any other reasons to keep the bomb?

It is said that maintaining Britain's position as a member of the nuclear "club" lends weight to the UK's status as a leading world power on the diplomatic stage and boosts the country's political prestige. Defence ministers have pointed to the importance of maintaining Britain's submarine-making industry, with Lord Drayson, the minister for defence procurement, saying only this week that industry needed a "steady drumbeat" of new orders for nuclear powered submarines. He stressed, however, that a decision on a Trident replacement would not be made on industrial grounds.

Why get rid of the bomb now?

Anti-nuclear campaigners argue that it is immoral to hold nuclear weapons at all, even if there is no real intention to use them. They claim that Britain loses its moral authority to tell states such as Iran or North Korea not to develop their own nuclear weapons if it and other holders of atomic weapons maintain and replace their stocks.

Some prominent international lawyers, such as the QC Phillipe Sands, have argued that replacing Trident would breach Britain's obligations under the Non-Proliferation Treaty. And opponents of nuclear weapons have also long claimed that a decision to end Britain's use of nuclear weapons would help spur other states to make progress on disarmament.

Where do the parties stand?

Labour's general election manifesto said it was committed to "retaining Britain's independent nuclear deterrent", however, there is a strong anti-nuclear lobby in the Labour ranks. The Liberal Democrats are reviewing their policy; it has backed retaining Britain's nuclear deterrent in the past, but it has strong anti-nuclear feelings among its rank and file. The Conservatives remain strong supporters of Britain's nuclear deterrent.

Do we have to keep the bomb?

Yes...

* Britain needs the bomb to deter the continuing threat from holders of the world's current stockpiles of nuclear weapons.

* Fresh threats could emerge from the development of nuclear weapons by unstable 'rogue states'

* Membership of the nuclear 'club' helps maintain Britain's status as a global diplomatic power

No...

* Holding nuclear weapons is of little use in a post-Cold War world, where the main threat comes from terrorism

* Renewing Britain's nuclear arsenal would undermine efforts to persuade other nations not to develop them

* A decision to replace Trident could breach international non-proliferation treaties to which Britain is a signatory