Prime Minister Tony Blair laid out plans Monday for a new multibillion-dollar submarine-based nuclear arsenal, warning lawmakers the future may hold perilous threats from rogue regimes and state-sponsored terrorists.

In what is expected to be among his last major acts as premier, Blair told the House of Commons that despite the end of the Cold War, potential threats were posed by North Korea, Iran and others.

"In these circumstances, it would be unwise and dangerous for Britain alone of any of the nuclear powers to give up its independent nuclear deterrent," he said.

Blair said Britain would cut back on its stock of nuclear warheads from 200 to 160 — a move intended to make the proposal more acceptable to detractors within his own party. But he said any decision to reduce the nuclear-armed submarine fleet from four to three would be made only after a new vessel is designed. Blair said advisers had ruled out land or air based alternatives.

Blair also said a decision on whether Britain will build a new arsenal of warheads to replace current stocks — expected to last only until the 2020s — would not be made before 2009.

The existing submarine fleet will be phased out from 2022, defense officials said.

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Britain's conservative opposition leader, David Cameron, endorsed the new deterrent, though he urged options be kept open for fourth submarine.

The replacement fleet would cost around $40 billion.

Blair said Britain would join in a U.S. program to extend the Trident D5 missile, currently used by both countries, until the early 2040s. President Bush has assured Blair that Britain would also be included in designing a successor missile, defense officials said.

Blair told legislators they would be asked to decide on the number of new submarines and a new missile design in the next Parliament, following national elections expected in 2009.

Kate Hudson, chairwoman of the Campaign for Nuclear Disarmament, said Iran and North Korea would see Blair's decision as a vindication of their own aspirations.

At the submarine fleet's naval base in Faslane, Scotland, around 100 campaigners protested Blair's plans.

Lawmakers will be asked to vote on the proposal by March, allowing three months for debate.

Britain's first Trident submarine, the HMS Vanguard, went on a maiden patrol in December 1994.

Opponents of a replacement system — including nuclear disarmament campaigners — claim the new fleet is likely to cost as much as $150.6 billion.

Britain's third party Liberal Democrats have argued that a decision could be put off until as late as 2014. The government claims a decision is urgent because it will take up to 14 years to design, commission and build the submarines.

A survey of members of the House of Commons indicated that Blair faces strong resistance within his party.

Among 80 Labour lawmakers who completed a questionnaire, 39 percent were opposed to a new generation of nuclear-armed submarines, CommunicateResearch reported for The Independent newspaper.

In contrast, 94 percent of 52 Conservatives supported a new generation.

Beyond five formally declared nuclear weapons states — the United States, Russia, China, France and Britain — four others are known or thought to have such arms. They are India, Pakistan, Israel and, following its October missile test, North Korea.

Of the nuclear powers, the United States, Russia, France and Britain currently have submarine, air and land-based capability.