Tony Blair has argued that Britain needs to buy a new generation of nuclear weapons because it might not be able to rely on the United States to protect it if it were attacked. The Prime Minister, who flies to Washington tomorrow to discuss an exit strategy from Iraq with George Bush, surprised MPs by suggesting Britain could not take America's support for granted as he announced the Government was backing a submarine-based "son of Trident" system.

"Our co-operation with America is very close. But close as it is, the independent nature of the British deterrent is an additional insurance against circumstances where we are threatened but America is not," said Mr Blair. "These circumstances are also highly unlikely but I am unwilling to say they are non-existent."

Critics claim the British deterrent is not independent because Trident's D5 missiles are made in America. While new submarines will almost certainly be built in Britain, the UK and America will work jointly on a new missile. Mr Blair insisted the submarines, missiles, warheads and command chain would be entirely under British control.

The Prime Minister said it would be "unwise and dangerous" in an uncertain world for Britain to surrender its "ultimate insurance". He said there was a new and potentially hazardous threat from countries such as North Korea and Iran which were developing nuclear weapons and that rogue states would sponsor nuclear terrorism. Although the British deterrent would not deter terrorists, he admitted, it might have an impact on governments connected to them.

"In the final analysis, the risk of giving up something that has been one of the mainstays of our security since the war, and moreover doing so when the one certain thing about our world today is its uncertainty, is not a risk I feel we can responsibly take," he said.

Mr Blair denied that replacing Trident would breach the Nuclear Non-Proliferation Treaty and sought to head off a Labour backbench rebellion by saying that Britain's stockpile of warheads could be cut from 200 to 160 and the number of submarines reduced from four to three.

He put the cost of the new system at between £15bn and £20bn. But there would also be running costs of more than £1.5bn a year, raising the total cost to more than £65bn over 30 years.

Labour MPs opposed to a new Trident claimed the Cabinet had been " bounced" into supporting it after ministers rubber-stamped yesterday's White Paper only three hours before it was published. Downing Street insisted there were no dissenting voices and that ministers' views had been taken into account.

Margaret Beckett, the Foreign Secretary, said: "I thought it through very carefully and with deep reluctance came to the conclusion that probably it is an even more unsafe world than it was and not therefore a world in which it is sensible for us now to abandon our nuclear deterrent."

In the Commons, some Labour MPs expressed alarm. But Mr Blair was assured of victory when MPs vote on the White Paper in March when David Cameron assured him of the support of the Tory Opposition. He questioned whether the plan to cut the number of warheads might go too far.

Michael Meacher, the former Labour minister, asked: "How can this proposal be justified in a post-Cold War environment?" He warned it would "severely restrict" expenditure on conventional defence and issues such as terrorism, climate change and long-term energy and security.

Michael Ancram, the former shadow foreign secretary, asked Mr Blair: " Are we seriously to believe that we would ever use this most potent nuclear weapon against rogue states or terrorist organisations? Why don't you explore more credible non-nuclear alternatives."

Sir Menzies Campbell, the Liberal Democrat leader, argued that there was no need to rush a decision which could be delayed until 2014. He asked Mr Blair: "Is this about Britain's interest or your legacy?"

The Campaign for Nuclear Disarmament said the billions earmarked for a Trident replacement should be spent on vital public services. Kate Hudson, CND's chairman, said she was "very, very disappointed" with Mr Blair and accused him of not listening to the public. "He talked vaguely about reducing the number of submarines and warheads but it is not clear what that would mean," she said.

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