Parties still at war over Trident

Almost 20 years since the end of the Cold War, the issue of Britain's nuclear deterrent remains an emotive one.

Government plans to update and replace the Trident nuclear weapons system, including its four Vanguard submarines, were approved by MPs in 2007 - but only after a huge Labour rebellion left ministers relying on Conservative support.

Now Gordon Brown is proposing that Britain might cut its number of submarines from four to three if it would help secure a deal to dramatically reduce the world's nuclear arsenal.

Military chiefs have already been exploring the possibility of a three-submarine fleet, which would need to guarantee adequate deterrent capability, and the idea has also received backing from the Tories.

As the global recession has led all parties to examine ways to cut costs, the £20 billion bill for replacing Trident - which is expected to go out of commission some time in the 2020s - has naturally come under the spotlight.

Greenpeace has warned that the total cost of building and operating a replacement could reach £97 billion, though this has been rejected by the Ministry of Defence.

While the Conservatives have committed themselves to renewing the deterrent, shadow defence secretary Liam Fox indicated earlier this year that a fleet of three submarines might provide better value-for-money for the taxpayer.

Another option for politicians and commanders is whether the life of the existing submarines, missiles and warheads could be extended to save money.

But opposition to replacing Trident is not based on cost alone.

A poll earlier this year suggested 54% of Britons wanted the deterrent scrapped, while a group of retired senior military officers branded the system "completely useless" against the threats of the modern world.

Liberal Democrat leader Nick Clegg has argued that such a powerful nuclear weapon is not needed in the post-Cold War age, while also suggesting Britain can no longer afford it

A vocal anti-nuclear lobby is also opposed to replacing Trident on principle, and features the Scottish National Party among it.

The UK's nuclear submarines currently operate out of Faslane on the Clyde, but the SNP - in power at Holyrood - argues that the weapons should not be based in Scotland and has set up an anti-Trident working group.

Legal questions have also been raised as to whether replacing Trident would breach Britain's obligations under the Nuclear Non-Proliferation Treaty.