Britain's current nuclear deterrent consists of four Vanguard submarines, each capable of carrying up to 16 nuclear missiles. Britain's Defense Ministry says that fleet will become obsolete by 2024.

Tony Blair, 14 Mar 2007

"I think that is essential for our security in an uncertain world and I believe that it is important that we recognize that although it is impossible to predict the future, the one thing as I said in my statement that is certain, is the unpredictability of it and for that reason I think that it is sensible that we take this decision today," he said.

Critics, including many in Mr. Blair's own ruling Labor Party, say with the end of the Cold War there should be no place for nuclear weapons in Britain's arsenal. They contend an upgrade of Britain's current nuclear weapons system would actually send the world the wrong signal.

Mr. Blair disagrees.

"I think what is possible is for us to continue to play our full part under the Non-Proliferation Treaty in the multilateral negotiations that I hope will take place over the years to come so that the world becomes a safer place with fewer nuclear weapons," he said. "But that is something that I think that we are best able to achieve if we maintain our nuclear deterrent."

Among those against upgrading Britain's nuclear weapons are many who say the decision does not have to come right now. They point out that Mr. Blair will be out of office in the next few months, and gauging whether to hold nuclear weapons might better be assessed in a few years.

But Mr. Blair told the House of Commons preparations for the rearmament program must begin now.

"I entirely understand why people may want to put off this decision but the fact is that if we want to get parliamentary approval for the work that has to begin now on the concept and design phase of this - of course the actual contracts for the design and construction are let at a later time - but it is today that we need to take the decision for the concept and design stage of this and if we want to get proper parliamentary authorization for this, this decision has to be taken now," he said.

The British government estimates the new submarines and the upgraded nuclear missiles would cost at least $40 billion.