The Big Question: Is scrapping one of Britain's four Trident nuclear submarines sensible?

Why are we asking this now?

Because the Prime Minister is to tell the United Nations today that he is willing to reduce Britain's fleet of Trident missile-carrying submarines by a quarter – from four down to three.

Could we have an effective deterrent with just three?

He is not planning to reduce Britain's warheads further from the 160 it now stands at (having been at 200) but merely to deploy them in fewer submarines. Since each warhead is eight times more powerful than the bomb that destroyed Hiroshima, that remains pretty hefty firepower. Britain has one submarine armed and at sea at all times. Another is undergoing maintenance. Another is engaged in training exercises. The fourth is a "spare", docked at port.

Some, like the former Defence Secretary John Hutton, fear that losing one boat would jeopardise round-the-clock underwater patrols. "It would be like having an insurance policy that only covered you for three-quarters of the time," he said yesterday. But other experts, including Professor Ron Smith, a defence economist at Birkbeck College, say that the cut would probably have little effect on Britain's nuclear capability.

How long will the existing system last?

The submarine-launched Trident system, which has provided Britain's nuclear cover since 1994, will start coming to the end of its life in 2024. But a replacement could take more than a decade to commission, design and build. So decisions on what should replace Trident need to be taken fairly soon.

The United States has decided to extend the life of its Trident submarines so they will be effective until 2040 or a decade longer. Some argue that the UK should do the same to buy time to allow for a major rethink of British nuclear strategy.

Is this about saving money?

In part. The Government is keen not just to find ways to save money but to find ways of being seen to do so. Trident is a symbolic "Big Spend" at a time when vital public services are to be cut. But losing one of four submarines would not result in a 25 per cent saving. That is because there are a lot of upfront fixed costs and there are economies of scale, with each boat costing less to build than the last one. Cutting one sub would save only "a couple of billion".

More importantly, nuclear defence is a long-term strategic decision rather than one which should be subordinated to the exigencies of a comparatively short-term economic cycle – even one as disastrous as the current credit crunch crisis.

Or about promoting the non-proliferation of nuclear weapons?

It is significant that Mr Brown is floating his suggestion at a meeting of the UN Security Council held to discuss halting the spread of nuclear weapons and reducing existing stockpiles.

It will be chaired by Barack Obama who has indicated that he wants Washington and Moscow to agree a reduction of their nuclear warheads from the current levels of 2,800 Russian warheads and 2,200 American ones.

But the meeting also aims to stop new countries from developing nuclear weapons. Britain wants to help set the agenda for that and the proposed cut is supposed to set a good example.

How would the change affect our defence industry?

It will have some impact, but not a decisive one. But not replacing Trident at all could severely damage Britain's submarine and nuclear industries.

That could jeopardise 15,000 jobs – and risk losing considerable expertise. But it is improbable that a decision on a Trident renewal would be taken solely on industrial grounds.

How would it impact on our international status?

All of the permanent members of the UN Security Council – the US, Russia, France, China and Britain – are nuclear powers. Retaining or replacing Trident keeps Britain a member of that "nuclear club", gives it more clout within the diplomatic community and boosts the country's international standing.

But it is possible to make reductions without fundamentally altering that. Britain has lowered the explosive capacity of its nuclear weapons by 70 per cent since the end of the Cold War (and has also "de-targetted" its missiles so they don't point at targets in Russia).

Before 1998, the UK had 300 warheads (as France still has) but this has been cut to 144 operational devices with another 15 per cent as back-up.

This compares with an estimated 180 Chinese warheads and 80 in Israel, 60 each in India and Pakistan and 10 in North Korea. So, though Britain is undoubtedly in long-term decline as an international power, it continues to punch above its weight diplomatically.

Why don't we just rely on the United States?

We already do. Britain has not had an independent weapons delivery programme since the cancellation of the Blue Streak in the 1960s. The UK is, technically, dependent on the US. British Trident warhead components are made in the US and its missiles are serviced there.

Why don't we just scrap the whole lot?

Many people think that we should. They say that nuclear weapons are immoral, impractical and unaffordable in the modern age. The Cold War threat from the Soviet Union is over and nuclear weapons are useless against the new threats that are posed by international terrorism.

But others argue that giving up nuclear weapons is not like giving up smoking, "something that any strong-minded state ought to be able to accomplish whatever anyone else may do", as the high priest of nuclear deterrence, the late Sir Michael Quinlan, once put it. It is only sensible to give up nuclear weapons if everyone else does – and, at present, the world is moving in the opposite direction.

Moreover, future perils may be different from the current terrorist menace. Russia could become destabilised. China could become unpredictable as its power grows. The intentions of North Korea and Iran are opaque.

The world "needs to contemplate not just a nuclear rogue state amid global disapproval," said Michael Quinlan, but the prospect of "a one-sidedly nuclear Hitler, Stalin, Pol Pot or Saddam Hussein at war with us in an anarchic world". So it would be madness for "responsible powers" to get rid of their weapons just as unstable or rogue states are acquiring them.

"The nuclear dragon," says Sir Michael Howard, Professor of the History of War at Oxford, "is not dead, but sleeping."

Should Britain dismantle its nuclear capability altogether?

Yes...

* International agreement on scrapping nuclear weapons, and blocking new ones, should be encouraged

* With the Cold War over, nuclear weapons are of little use if the main threat to us comes from terrorism

* In the present economic climate, Britain can't afford to replace Trident on a like-for-like basis

No...

* It's madness for responsible powers to give up nuclear weapons just as rogue states are developing them

* In the next few decades, fresh threats could come from states such as Russia and China

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