The life of Britain's D5 Trident nuclear missiles, which are also used by the US, can be extended until 2042, the Prime Minister said, so no decision was needed on their future was needed now.

But he added that he would be writing to President Bush to seek American collaboration in developing the next generation of missiles.

In a concession to Labour backbenchers who oppose Britain's nuclear capabilities, he said that the fleet might only need three submarines, but that a final decision would depend on their design. Two submarines would not be enough to ensure that one is at sea at all times.

In another offer to rebel MPs, Mr Blair said the plan also envisaged a 20 per cent reduction in the UK's stockpile of nuclear warheads: from 200 to 160.

"Ultimately, this decision is a judgment, a judgment about possible risks to our country and its security; and the place of the deterrent in thwarting those risks," he told the House Commons.

"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 interests will not emerge."

Mr Blair said that the nuclear ambitions of countries such as North Korea and Iran and the potential connections between those regimes and international terrorism meant that it would "be unwise and dangerous for Britain, alone of any of the nuclear powers, to give up its independent nuclear deterrent".

He criticised as naive the thought that Britain could persuade other countries to disarm by abandoning its nuclear programme. "More likely, they would construe it as weakness," he said.

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