Ministers will bring forward proposals on a successor to Trident before the end of the year, which will be followed by a "period of debate", the spokesman told reporters.

But the vote, expected early next year, will be "whipped" and not free, said Jack Straw, who as Leader of the House of Commons is in charge of managing government business in the lower chamber.

Whipping is a system where lawmakers are coerced to not only attend a vote on a major issue but to back the party line.

A free vote makes it easier for them to follow their consciences.

The decision to whip the vote is likely to prove controversial with more than 120 Labour lawmakers having lobbied ministers to let them have a say on the issue, and raises the prospect of a backbench rebellion against Blair.

Trident has proved to be divisive issue for the ruling Labour Party under the pro-nuclear, modernising Blair, who became leader in 1994.

Opposition to nuclear weapons and power was historically a central plank of Labour Party policy and disagreement over the current position underlines differences between left-wing "old" and centre-left "new" Labour.

In the 1980s, party leaders such as Neil Kinnock spoke at marches organised by the Campaign for Nuclear Disarmament (CND).

Blair has indicated he wants to keep the nuclear deterrent, as has his probable successor, finance minister Gordon Brown.

But senior ministers including Foreign Secretary Margaret Beckett reportedly have concerns about Trident.

The main opposition Conservative Party believes in keeping Trident, while the second opposition party, the Liberal Democrats, wants to retain a minimum deterrent "for the foreseeable future".

Britain's aging nuclear deterrent consists of four Royal Navy submarines, one of which is always on patrol, fitted with US-built Trident missiles.

It will become obsolete in the mid-2020s. A successor would require many years of development and could cost up to 25 billion pounds (37 billion euros, 46 billion dollars), observers say.

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