Tony Blair said yesterday it would be 'dangerous' for Britain not to hve a nuclear deterrent as he unveiled plans to upgrade the ageing nuclear missile system, Trident. Lindsay Jennings looks at the key issues surounding the debate.

IT has been seen by some as the chance for Britain to take the moral high ground. As more and more countries across the world step up their efforts to join the nuclear club - including so-called rogue states North Korea and Iran - Britain could decommission its ageing nuclear missile system, Trident.

There would be no more calls of hypocrisy over Britain's attempts to clamp down on other nations developing nuclear capabilities, and there would be more cash to spend on tackling issues such as global warming, anti-nuclear campaigners argue.

But according to Tony Blair, we would also be leaving ourselves wide open to a "major nuclear threat".

As the Government unveiled its White Paper to build a new generation of submarines to carry Trident missiles, we look at the arguments for and against upgrading the system.

Q What is Trident and why does it need replacing?

Prime Minister Margaret Thatcher acquired Trident from the Americans in 1980 to replace Britain's previous deterrent, Polaris. The first Vanguard-class submarine carrying the missile started patrolling in 1994 and there are now four of them based at the high-security Faslane naval based on the Clyde in Scotland. Trident is entirely sea based with three elements - the submarines, the missiles and the nuclear war heads.

The Ministry of Defence says that the four submarines only have an operations life of 25 years - meaning that the first submarine will need to be withdrawn in 2019 and the last in 2026.

Q Why do we have to start upgrading now - can't we make a decision later?

The Government argues it will take up to 14 years to design, commission and build replacement submarines. The American-made D5 missiles can be extended into the 2040s under a US modernisation programme. Meanwhile, the warheads, which are made in Britain, are expected to be maintained at least into the 2020s with work carried out at Aldermaston, in Berkshire, with investment which was announced in July last year.

But Liberal Democrat leader, Sir Menzies Campbell would like to put the decision making off until 2014. He says that we will have a better idea of the threats we face then.

Q Why do we need a nuclear deterrent? The Cold War's over, isn't it?

It is true that Trident was designed for the Cold War. But according to the Government, there is no way to predict what security threats we may face 20 or 30 years from now with states such as Israel, India and Pakistan in possession of nuclear weapons and North Korea and Iran intent on developing them.

But Dr Kyle Grayson, lecturer in international politics at Newcastle University, says that even though the Cold War is over, relationships with Russia, in particular, have been put under strain with the recent poisoning of Russian former spy Alexander Litvinenko.

"What the Government seems to be saying is 'better safe than sorry'," he says. "They're saying it's a direct response primarily to the rogue states of North Korea and Iran, but it makes little sense to think the UK would be directly threatened by North Korea and Iran has made it pretty clear that it wants a nuclear option that has everything to do with Israel and less to do with the UK. I believe the conversations that are going on behind closed doors will be more about the threat of Russia and China."

Q Shouldn't we be spending the money on our hospitals or alleviating Third World poverty?

Anti-nuclear campaigners certainly think so. They argue that the money it will cost to upgrade Trident - between £20bn and £70bn - would be better spent on tackling global warming, for example. They say that Trident was designed for the Cold War era and is not relevant today.

Other options would be to spend the money on expanding the British Army - ensuring its soldiers have enough up-to-date equipment from flak jackets to armoured cars and light tanks - and putting more money into counter intelligence to combat international terrorism.

There are arguments over whether Trident would be obsolete in 30 years time, even with an upgrade, as the spread of satellite technology means submarines may soon be detectable from space.

The US is also close to designing a missile defence system, says Dr Grayson. "What's to say in 30 years other countries won't have that?" he adds.

Q Doesn't renewing Trident undermine our diplomacy when we're trying to limit the number of countries developing nuclear weapons?

It has been considered hypocritical by some to urge North Korea and Iran to abandon nuclear ambitions when we are intent on upgrading our own system.

Under the Nuclear Non-Proliferation Treaty, signed by Britain in 1968, Britain is one of just five states - alongside the US, Russia, China and France - permitted to own nuclear weapons. The treaty commits countries to work towards reducing, and eventually eliminating, their own stockpiles.

The Government argues that the treaty does not commit member states to total disarmament but only to negotiations on effective measures and that it has fulfilled this pledge.

It announced yesterday it intended to cut the number of Vanguard submarines from four to three and the number of nuclear warheads by 20 per cent.

But the treaty also forbids the transfer of nuclear weapons, even between nuclear states, which Britain and America are breaking, according to critics. The two countries have shared nuclear weapons technology and materials since July 1958 when the Mutual Defence Agreement was signed.

Q So, what are the options if we don't replace it?

One option would be for the US to offer an "umbrella of deterrent". But this is unlikely given the current political situation, says Dr Grayson.

"Politically that's not going to play well and it's certainly not going to make Labour look any less dependent upon the US than they are already perceived to be," he says.

The other option is to do nothing, and make a grand global statement by opting out of the nuclear club entirely.

"There's a significant amount of national prestige and status that comes with being a nuclear power," says Dr Grayson. "But if the UK swore against its deterrent it would be putting itself forward as a significant moral voice in global politics. It would also get rid of a lot of hypocrisy."

Q What happens now? Will the vote in the House of Commons go the Government's way?

Almost certainly. Although dozens of Labour MPs may rebel when the Commons votes in March next year, the vast majority of Conservative MPs are expected to back the replacement of Trident.