Britain's Tony Blair launched plans for a multibillion dollar nuclear missile defense system Monday, warning lawmakers the future would likely hold hazardous threats from rogue regimes and state-sponsored terrorists. Blair told Britain's House of Commons the end of the Cold War had brought no guarantees a new "major nuclear threat" would be curbed, citing the weapons ambitions of North Korea and Iran as proof of the enduring need for a defense system. "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," the British prime minister said. Britain would not need the "terrifying power of these weapons unless we believed that to be necessary to deter a future aggressor," Blair told legislators, setting out proposals for a new submarine-borne missile system. Blair said the country's stock of nuclear warheads would be cut from 200 to 160 - a move aimed at placating detractors within his own party. But he said a decision on whether to reduce the nuclear-armed submarine fleet from four to three would be taken only once a new vessel is designed. Blair said advisers had ruled out land or air based alternatives as too costly and too vulnerable. A decision on whether Britain will build a new arsenal of nuclear warheads to replace current stocks - expected to remain in service until the 2020s - would not be taken before 2009, he said. It meant Blair - due to stand down in 2007 and making one of his last major proposals as prime minister - avoided the most contentious aspect of committing to a future nuclear defense program. Debates over a new warhead program are expected to stir up fierce divisions in his Labour party, once committed to unilateral nuclear disarmament Britain's existing submarine fleet will be phased out from 2022 and replacement vessels will take 17 years to build, defense officials said. Britain's opposition Conservative party leader, David Cameron, endorsed the new deterrent, which Blair said would cost around 20 billion pounds, (US$40billion; â‚¬30billion). Blair said Britain and the US would extend the life of the Trident D5 missile, used by both countries and carried on British submarines, until the early 2040s. US President George W. Bush has sent a written assurance that Britain would also be included in designing a successor missile, defense officials said. At the submarine fleet's naval base in Fasten, Scotland, around 100 campaigners held a rally to protest Blair's decision. 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. Third party Liberal Democrats leader Menzies Campbell questioned the timing of the decision. "Is this about Britain's interest or your legacy?" he asked Blair in the Commons. A senior Foreign Office diplomat, speaking on condition of anonymity because he was not authorized to issue a public statement, said officials believed increased nuclear proliferation and technologically sophisticated terrorism made nuclear-armed terror groups almost inevitable. "We know there is a desire among some terrorist groups to acquire nuclear weapons," he said. Lawmakers will be asked to vote on the proposal by March 2007 and senior officials said the plan was expected to win overwhelming support. However, a survey of legislators indicated Blair faces some 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 the plan. 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. Both the US and Russia maintain submarine, air and land-based capability. France has submarine and air-launched systems and China has a small land based deterrent, but is designing a submarine based missile. Britain maintains a single, submarine-borne deterrent and has the lowest number of warheads among the five, Blair's office said.