The motion only passed with the backing of opposition Conservatives, after 95 Labour MPs voted against their own government, according to the BBC.

The parliamentary rebuff was believed to be the the biggest rebellion within Labour ranks since March 2003, when 138 Labour MPs, including former foreign secretary Robin Cook, voted against invading Iraq.

Anti-nuclear campaigners hailed the vote as a success with Kate Hudson of the Campaign for Nuclear Disarmament (CND) describing it as a "major victory for the peace movement."

The leader of the smaller opposition Liberal Democrats, Menzies Campbell, who opposed the decision, said it was a "humiliation" for the government.

"I think the government will be well advised to take account not only of the vote but also of public opinion which is very very doubtful" about the Trident renewal plans, he added.

Blair is widely perceived as gradually losing his grip on party discipline as he prepares to step down in the coming months after a decade in power. He is expected to hand over to finance minister Gordon Brown.

Before lawmakers debated the issue, Blair told them a future parliament could still decide in 2012-2014 whether or not to put out new contracts for new nuclear submarines that carry the weapons.

But it was vital to begin the concept and design process on the vessels immediately as they took 17 years to develop and the four existing submarines were expected to reach the end of their working life by 2024.

"I think it's right we take the decision now to begin work on replacing the Trident nuclear submarines. I think that is essential for our security in an uncertain world," Blair said.

"I believe it is important that we recognise that although it is impossible to predict the future, the one thing ... that is certain, is the unpredictability of it.

"For that reason, I think it is sensible we take this decision today."

Blair outlined proposals in December to replace the submarines and extend the shelf-life of the weapons beyond 2050, sparking widespread opposition from the traditional anti-nuclear lobby and many in his own Labour party.

Many Labour MPs had indicated they would go against the government over Trident while four junior ministerial aides this week resigned in protest -- including two during the debate itself.

Opponents, including Church leaders and unions, dispute government claims it would cost 15-20 billion pounds (22-29.1 billion euros, 29-38.5 billion dollars) to replace.

They argue the cost could rise to more than 100 billion pounds if maintenance and other costs are added, that Trident is a Cold War relic and the money could be better spent elsewhere.

Opponents say most Britons are against renewing Trident and that in doing so Britain is being hypocritical because of its attempts to prevent Iran and North Korea obtaining nuclear weapons.

It could also potentially trigger a new wave of nuclear proliferation, they add.

Within Labour, where unilateral nuclear disarmament was party policy in the 1980s, grassroots members and left-wing traditionalists in parliament have accused Blair of stifling debate on the proposals.

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