MPs have voted in favour of spending £20bn on replacing the UK's nuclear missiles.

After hours of debate in the Commons, 409 MPs voted to renew the nuclear deterrent, with 161 against.

The Prime Minister said it was urgent that work started straight away on replacing Trident in order to protect the country.

Tony Blair told MPs: "I think that is essential for our security in an uncertain world."

North Wiltshire MP James Gray, who supported the Prime Minister, said it was "quite wrong" to assume the country could lead the way by getting rid of weapons.

He said: "It seems to me quite obvious that we are the good guys and they are not and therefore it is perfectly reasonable that we should have these weapons."

Peace campaigners protested all day outside Parliament.

Swindon Campaign for Nuclear Disarmament (CND) secretary Andy Newman said: "The nuclear deterrent is not credible because the destruction that would be unleashed would be so unimaginable that no sane government would use it.

"Any use of nuclear weapons against another nuclear power would be suicidal.

"Any use of nuclear weapons against a non-nuclear power would irrevocably damage Britain's international standing and economy, as well as possibly provoke terrorist responses we could not defend against.

"Other G8 countries such as Japan, Germany, Italy and Canada have no nuclear weapons, and have no plans to develop them. Why does the UK need weapons that these countries do not?"

The peace group said replacing Trident would cost about £25bn to start with and at least £75bn once maintenance and repair is taken into account, and said the money could be better spent on education or health care.

Mike Haslam, vicar of North Swindon, said he was worried replacing Trident could destroy the environment.

He said: "While our armed forces are underpaid and under-equipped; our public services continue to need to be resourced; our environment is in desperate need of protection, and there is growing agreement that environmental catastrophe is the greatest danger now facing humanity, to spend such billions on nuclear weapons is wrong."

Mr Blair is expected to come under fire as around 90 Labour MPs rebelled against the party line last night.

Conservative leader David Cameron and many Tory MPs helped push the vote through.

Mr Cameron said: "Replacing Britain's nuclear deterrent is clearly in the national interest. The decision needs to be taken now."

In an earlier vote, MPs voted 413 to 167 against delaying a decision until 2014.