Defence Secretary Liam Fox has given the go-ahead for initial work to begin on the replacement of the UK's Trident nuclear weapons system.
In a Commons statement on 18 May 2011, Dr Fox told MPs that the government had approved the first stage of design for replacement submarines with an initial outlay estimated at £3bn.
Trident was the "ultimate guarantee of national security", he said.
Dr Fox said that the plan would cost up to £25bn and could still be operational in the 2060s.
He described the move as the "initial gate" decision, paving the way for detailed plans to be drawn up ahead of the "main gate decision", due in 2016.
Dr Fox said that while no nuclear-armed nation currently posed a danger to the UK, ministers had to think ahead.
"We simply do not know how the international environment will change in the next few years, let alone the next 50 years," he said.
But he added that Britain's long-term goal remained a world without nuclear weapons and said the UK's nuclear armoury would decrease from a maximum of 225 missiles to 180, and a maximum number of warheads from 48 to 40 on each submarine.
Shadow defence secretary Jim Murphy said uncertainty over potential nuclear weapons in North Korea and Iran showed why replacing Trident was important.
He told MPs: "Britain's independent deterrent has been the cornerstone of our peace and security over half a century.
"As long as there are other countries with such capability it is right the UK retains an independent nuclear deterrent."
Dr Fox also announced that a review of different possible ways to replace Trident would be launched "to assist the Liberal Democrats in making the case for alternatives".
Former Lib Dem leader Sir Menzies Campbell urged the defence secretary to keep an open mind on the conclusions of the review.
Dr Fox replied: "We are committed as a government to the replacement of the Trident system and the spend will go ahead throughout the rest of this Parliament, as I have outlined."
But Labour backbencher Paul Flynn described the nuclear deterrent as a "national virility symbol".
His colleague Jeremy Corbyn concluded that its renewal would "make the world not a safer place but a more dangerous place".
Lib Dem MP Martin Horwood added that his party would campaign against Trident at the next general election.