"I think it's right we take the decision now to begin work on replacing the Trident nuclear submarines," he told lawmakers during his weekly question session in the House of Commons before the nuclear deterrent debate began.

"I think that is essential for our security in an uncertain world. 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 last December to replace the existing Vanguard-class nuclear submarines that carry the US-built Trident missile system and modernise the weapons themselves.

Military experts assessed that it takes 17 years to design and develop a new system and with the deterrent expected to become obsolete in 2024, parliamentary approval was needed now, he said.

The decision triggered widespread opposition from the traditional anti-nuclear lobby but as well as unions, religious groups and inside Blair's governing Labour, where unilateral nuclear disarmament was once party policy.

The biggest rebellion by rank-and-file Labour MPs since March 2003 over the decision to invade Iraq is predicted at the vote, which is due about 1900 GMT.

Blair is unlikely to lose because he has the support of the main opposition Conservative Party.

Tory leader David Cameron gave his renewed backing to Blair Wednesday, but Menzies Campbell, leader of the smaller Liberal Democrat Party, said such a link-up could be disastrous.

He said that last time Labour had to rely on the opposition benches to push a vote through was during the Iraq debate.