The UK needs to keep its nuclear weapons because of threats to future generations, Defence Secretary Des Browne has told MPs.
Des Browne said the UK had to have a 'credible' deterrent
He said maintaining Trident was the "overwhelmingly sensible" decision for MPs to make when they vote on the government's White Paper in March.
The cost of a new generation of nuclear submarines has been estimated at £20bn.
The Lib Dems want cut the number of Trident warheads by half and delay a decision on replacing it until 2014.
Appearing before the defence select committee, Mr Browne said that the White Paper had set out the nature of the threat that the country is "likely to and probably will" face in coming years.
"Once you accept that that threat is there, you commit to a deterrent," he told the MPs.
The submarines would take 17 years to develop and build, and would last until about 2050.
Trident is a submarine-based nuclear weapons system
Mr Browne said a decision had to be made now to maintain the deterrent because "history tells us that countries' intentions change very quickly".
The White Paper also said that the UK would join the US programme to extend the life of the Trident missiles until 2042 - and would then "work with" the US on successor missiles.
With the 300th operational patrol due to be completed this year, Mr Browne emphasised that to maintain a nuclear deterrent the UK had to be operationally independent.
He said independence was "jealously guarded" by the government.
Opponents of the plans argue that nuclear weapons are outdated after the end of the Cold War and would be useless in combating the threat of terrorism.
They also question whether the UK should be setting an example to stop proliferation of nuclear weapons around the world.
Mr Browne told the committee that having a deterrent was not an "outmoded concept", as it was key to the UK's defence policy.
"As far as we're concerned in this government, we are committed to maintaining a minimum deterrent and one credible to any potential aggressor."
Nuclear weapons would be used only in self-defence, and the defence of Nato allies, and only in extreme circumstances.
However, he said it was always defence policy to have some "ambiguity" about the exact circumstances under which nuclear weapons would be used.
Mr Browne agreed with a suggestion from the committee that the deterrent was "a political weapon, not a military weapon".
Mr Blair, launching the White Paper in December, said submarine numbers may be cut from four to three, while the number of nuclear warheads would be cut by 20%
However, Mr Browne said cutting the number of submarines was an "ambition", but the government was not sure that a deterrent could be maintained with less than four.
The Green Party is to hand in a letter to Downing Street on Wednesday calling for Trident to be scrapped.
Principal speaker Sian Berry said: "In what circumstances - and against whom - do the government think Trident will be used?
"Every pound wasted on nuclear weapons is a pound not spent on tackling climate change - the threat of which is very clear.
"How then can the government justify the staggering £76bn it is estimated this will cost?"