The Government has decided to acquire a new generation of nuclear missile submarines, Tony Blair has announced.
But he told MPs Britain's stock of nuclear warheads would be cut by 20% from 200 to 160.
The Prime Minister said the risk of a "major nuclear threat" meant it was essential to replace the Royal Navy's ageing fleet of Vanguard class submarines.
The estimated cost of replacing the fleet, which carries the Trident nuclear missiles, is £20bn.
The Government's White Paper has also left open the option of reducing the submarine fleet from four to three.
The White Paper was approved by the Cabinet earlier in the day.
Ministers have promised a Commons vote on the issue early next year.
With the Conservatives expected to back the Government, there is little doubt about the outcome.
But many Labour MPs remain deeply unhappy about the decision.
A poll for The Independent suggests as many as one in four is against retaining Trident.
In a forward to the White Paper, Mr Blair said other countries having big nuclear arsenals meant Britain could not afford to give up its deterrent.
"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."
He went on: "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."
:: Each warhead is British designed and made, and has the explosive power of up to 100 kilotonnes.
:: Such a blast would have eight times the power of the atomic bomb dropped on Hiroshima in 1945.
:: Missiles are launched underwater and, within two minutes, are travelling faster than 12,000 mph.
:: According to the Ministry Of Defence, each Vanguard submarine has a maximum of 16 missiles and a total of 48 warheads.
:: At least one Vanguard-class nuclear submarine is on operational patrol at all times.
:: The vessels are 150 metres long - over twice the length of a Boeing 747 - but are among the quietest in the world.
:: Their nuclear reactors make enough power to supply a small town and the submarines produce their own oxygen and fresh water.
Could you post the source (link) of the story please? Ta.
-Muezzin