Tony Blair says itis "essential" the
The prime minister may face his biggest Commons rebellion since the start ofthe
Up to 80 Labour MPs may rebel, but defeat in the 1900 GMT vote is unlikelyas the Conservatives back the plans.
Mr Blair told the Commons replacing Trident was "in the nationalinterest" and any delay would be "absurd".
Speaking during prime minister's questions before the debate got under way,Mr Blair said: "I think it's right we take the decision now to begin workon replacing the Trident nuclear submarines.
"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 iscertain, is the unpredictability of it."
But MPs including Liberal Democrat leader Sir Menzies Campbell and formerdefence minister Peter Kilfoyle are backing an amendment - thought to have thesupport of about 80 Labour MPs - saying the case for replacing Trident is"not yet proven" and saying they are "unconvinced of the needfor an early decision".
The Lib Dems want to postpone a decision until 2014, but the governmentargues that one needs to be made now as the submarines take so long to build.
Sir Menzies told the Commons: "A hasty decision to replace Trident isbound to undermine our ability to have influence at the conference [to reducenuclear proliferation] in 2010."
'Design and concept'
However, Mr Blair said: "It's absurd to suggest we could simply put offthe decision."
Some Labour MPs want an assurance that Parliament will be able to revisitthe Trident issue in the future.
Mr Blair said that was always a possibility, stressing that Wednesday's votewas for the "design and concept" stage, and Parliament might want torevisit the issue when it comes to contracts being negotiated for the work totake place from around 2012.
Mr Blair said in December that submarine numbers could be cutfrom four to three, while the number of nuclear warheads would be cut by 20%.
Between £15bn and £20bn would be spent on new submarines to carry theTrident missiles and the fleet would take 17 years to develop and build, andwould then last until about 2050.
The government's stance prompted two resignations this week - deputy Commonsleader Nigel Griffiths and ministerial aide Jim Devine both quit their posts inprotest.
'No enemy in sight'
Opening the debate, Foreign Secretary Margaret Beckett said large nucleararsenals still existed around the world and some countries in"unstable" regions were looking to create their own weapons.
She said: "There's the potential for a new nuclear threat to emerge orto re-emerge."
"Maintaining a nuclear deterrent remains a premium worth paying on aninsurance policy for this nation."
Former environment minister, and Labour leadership contender, MichaelMeacher is also against the plans.
"We're in the post cold-war environment, when there is no nuclear enemyin sight and the Ministry of Defence cannot actually suggest any nuclear enemyin the foreseeable future which might require nuclear weapons as asecurity."
He said the £20bn did not cover the ongoing costs of maintenance over 40years, which would end up "probably costing nearer £70bn".
Fellow would-be leader John McDonnell, who has also signed the amendment,said the vote would be a "defining moment" for the government.
Kate Hudson, chairwoman of CND, the anti-nuclear pressure group, said themotion's cross-party support indicated "the enormous unity that exists toprevent a rushed decision on Trident".
Labour rebels have staged a series of revolts since Tony Blair came to powerin 1997, although the government has suffered only four defeats.
The biggest rebellion was over
Several protests against replacing nuclear weapons are taking place acrossthe
Several demonstrators earlier climbed onto the roof of the ScottishParliament. Two people have been arrested.
BBCGüncelleme Tarihi: 20 Eylül 2018, 18:16