The plans include a new generation of nuclear submarines at a cost of up to 20 billion pounds (39.5 billion dollars).

In an apparent concession to critics from within his own party and the anti-nuclear lobby, Blair promised to cut the number of stockpiled nuclear warheads by 20 percent from about 200 currently to 160.

But he said: "The government's judgment, on balance, is that though the Cold War is over, we cannot be certain in the decades ahead that a major nuclear threat to our strategic interest will not emerge."

The premier, outlining the government's proposals to retain the US-built Trident missile system to parliament, said there were "new and potentially hazardous" threats from states like North Korea and Iran.

There was no guarantee that "there is a possible connection between some of those states and international terrorism" and that no other nuclear state in the world was considering unilaterally getting rid of its capability, he said.

"In these circumstances, it would be unwise and dangerous for Britain, alone of any of the nuclear powers, to give up its independent nuclear deterrent," he told the lower House of Commons.

Blair argued that action was needed immediately to take the first steps towards maintaining Trident, because of the estimated 17 years it takes to design build and deploy a new submarine.

A new generation of submarines would cost between 15 billion pounds and 20 billion pounds (22.2 billion-29.7 billion euros, 29.6 billion-39.5 billion dollars), including design and manufacturing costs, he added.

No decisions were needed on the warheads, as the lifespan of the Trident D5 missile can be extended to 2042, he said.

Blair rejected concerns that by retaining Trident, Britain was in breach of its obligations to nuclear non-proliferation.

"We have the smallest stockpile of nuclear warheads amongst the recognised nuclear weapons states, and are the only one to have reduced our stockpile of operationally available warheads to no more than 160, which represents a further 20 percent," he said.

"Compared with previous plans, we will have reduced the number of such weapons by nearly half."

The issue of replacing Trident is likely to dominate the political agenda in the coming months before a parliamentary vote on the matter in March.

Nuclear weapons are a divisive issue within Blair's governing Labour Party, as unilateral disarmament was a key plank of its policy at the height of the Cold War during the 1980s.

But he is unlikely to suffer an embarrassing defeat in parliament.

The leader of the main opposition Conservative Party, David Cameron, told Blair his party agreed with Blair's position "on substance and on timing".

Opponents, including the anti-nuclear lobby have claimed that Blair is trying to rush through a decision without a proper public debate and under pressure from the defence industry.

They also pointed to the cost of replacing the system, with observers putting estimates on anything from 25 billion pounds upwards.

The Trident system is currently based on four Vanguard class Royal Navy submarines, one of which is always on patrol and fully armed.

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