Tony Blair today gave the green light for a new generation of nuclear missile submarines, warning that it would be "unwise and dangerous" for Britain to give up its nuclear arsenal.
The Prime Minister told MPs that the new submarine fleet would carry the Trident nuclear missile deterrent into the middle of the 21st century at an estimated cost of £15 to £20 billion sterling.
In a concession to critics on the Labour benches, Mr Blair announced that the UK's stockpile of nuclear warheads would be cut by a fifth from around 200 to 160.
He also held out the prospect that technological developments may enable the size of the Trident submarine fleet to be reduced from four to three, cutting the overall cost of the programme.
However, on the principle of retaining the nuclear deterrent, Mr Blair was adamant that it remained "crucial" to Britain's national security.
Despite the end of Cold War, he said that the country faced a range of potential future threats - from the arsenals of existing nuclear powers to the prospect of rogue states sponsoring nuclear terrorism from within their territory.
"In these circumstances, it would be unwise and dangerous for Britain, alone among the nuclear powers, to give up its independent nuclear deterrent," he told the Commons.
With Tory leader David Cameron signalling his continued support for Trident, the Government is expected to win comfortably a Commons vote on the issue due to be held in March next year.
However the decision exposed deep divisions on the Labour benches, with many MPs deeply opposed to the retention of the deterrent beyond the lifetime of the current system.
Former minister Michael Meacher warned that it would "severely restrict" spending on conventional defence systems. "It will drain off colossal funds from areas like anti-terrorism, climate change and long-term energy and security issues," he said.
In a forward to the White Paper setting out the Government's proposals in detail, Mr Blair said that it was impossible to predict the security climate in 30 or 50 years time when the new system will be in service.
"We cannot be sure that a major nuclear threat to our vital interests will not emerge over the longer term," he said.
"I believe it is crucial that, for the foreseeable future, British Prime Ministers have the necessary assurance that no aggressor can escalate a crisis beyond UK control.
"These are not decisions a government takes lightly. The financial costs are substantial. We would not want to have the terrifying power of these weapons unless we believed that to be necessary to deter a future aggressor."
PA