TONY Blair was warned last night that he will be forced to rely on Tory MPs to force through £20bn plans for a new generation of Trident nuclear submarines.

Scores of Labour MPs have voiced opposition to renewing Britain's nuclear weapons system - potentially forcing the Prime Minister to fall back on Conservative support in a crunch vote in March.

Last night, Frank Cook, the Stockton North MP, predicted "a lot of unrest" on the Labour benches in what is shaping up to be one of Mr Blair's last parliamentary showdowns.

Mr Cook said: "I'm very concerned about it, because it's a very serious departure from our previous agreements.

"I've told the Labour whips I always vote with my conscience - and I disagree with weapons of mass destruction, whatever form they take."

Philip Cowley, an academic expert on Labour revolts, said: "I can't see how the Government can get this through. It will have to rely on the Conservatives and other opposition groups."

A poll yesterday found that 39 per cent of Labour MPs - and 31 per cent of MPs from all parties - believed Britain should not maintain a nuclear deterrent "for the forseeable future".

The warning came after Mr Blair, as expected, announced the Government's intention to spend £15bn-20bn on new Vanguard submarines, which carry the US-made Trident missiles.

In a statement to MPs, the Prime Minister insisted it would be "unwise and dangerous" for Britain - faced with the twin threats of rogue states and international terrorism - to give up its nuclear deterrent.

He announced the number of nuclear warheads would be cut by 20 per cent to 160 and suggested the number of submarines might be reduced from four to three.

But, Mr Blair insisted: "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."

There will be a three-month consultation, before a vote in March which - with the support of the Conservatives guaranteed yesterday - the Government is certain to win.

Mr Blair insisted a decision must be made next year, because two of the four nuclear submarines would become obsolete in 2024 and it would take 17 years to design and build a replacement.

However, yesterday's announcement does not involve ordering new D5 Trident missiles, because Britain will join an existing US programme to extend them to 2042.

Britain makes the warheads, with a decision on replacing those likely to be needed in the next parliament - creating a fresh flashpoint.

Kevan Jones, Labour MP for Durham North and a member of the Commons defence select committee, backed the Government's decision and predicted most Labour MPs would do the same.

Mr Jones said: "The Government is not replacing the warheads, but joining the US programmes to extend the life of the missiles through to 2042, which puts off a decision.

"Some people are unilateralists and I respect their position, but most people are concerned about the cost. This option means it won't cost up to £70bn, as some people feared."

However, Phil Willis, Liberal Democrat MP for Harrogate and Knaresborough, accused Mr Blair of a "mock consultation" when his mind was already made up.

He said: "This is shaping up to be a repeat of the 2002 Iraq debacle, when we were also told to trust the Prime Minister because he had been advised."