Charles Clarke, the former cabinet minister, has put himself at the head of the Labour rebellion against plans to replace Britain's Trident nuclear weapons system.
In a speech last night, Mr Clarke said the Government was in danger of equipping the nation to "fight the last war" and the Cold War rather than the threats facing it in the 21st century.
The former home secretary told the Fabian Society that the Cabinet was rushing a decision it did not need to make for years. A White Paper is expected before Christmas and will be voted on by MPs after a three-month consultation exercise. Tony Blair and Gordon Brown have signalled their support for building a "son of Trident" submarine-based system, which could cost up to £30bn.
Mr Clarke called for a more flexible and outward-looking approach to Britain's national security to reflect the changing nature of the many and varied threats it faced over the next 15 to 20 years.
While welcoming the forthcoming debate on the issue, he was "extremely sceptical" of the need to take an early decision and to replace Trident. "It is a very expensive weapon system which was developed in the Cold War to meet the conditions of the Cold War which ended 17 years ago, and it is still capable of functioning fully for about another 15 years from now," Mr Clarke said.
"There is a strong case for prioritising our security spending on what are likely to be the main security threats we face in the future, rather than building new weapons to fight the last war."
MPs are gearing up for a battle over Britain's independent deterrent. Forty-two MPs have signed a Commons motion calling on ministers to publish all the possible options and their costs to ensure an informed debate.
Another motion attacks the Government for deciding its position before the consultation exercise and deplores moves to deny Labour backbenchers a free vote. Two separate motions, tabled by Labour and Tory MPs, call for at least a year-long discussion before MPs vote.
In his speech, entitled The World after Bush, Mr Clarke also called on the European Union to adopt more proactive and co-ordinated foreign and defence policies, notably on the Middle East, Iraq and Afghanistan.
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