Trident and its replacement are "non-negotiable", the Prime Minister has said after being asked by one of his backbenchers about the renewal of Britain's nuclear deterrent.

David Cameron told MPs during Prime Minister's Questions that Trident and its replacement "are an absolutely vital part of this nation's security".

Mr Cameron claimed Labour "want to crawl into Downing Street on the coat tails of the SNP and put our country at risk".

Tory Julian Lewis (New Forest East) had said: "The Scottish National Party have been licking their lips in public at the prospect of blackmailing one of the two main parties into delaying or abandoning the replacement of the Trident submarine.

"Will the Prime Minister confirm that if he is still prime minister in 2016, as he should be, that he will certainly ensure that the Main Gate contracts for four successor submarines are signed that year?"

Mr Cameron responded: "I can reassure him. For me, Trident and its replacement are non-negotiable. They are an absolutely vital part of this nation's security.

"And let me just remind the party opposite this is the leaflet going out across Scotland and it says this: 'At the general election we need to stop the Tories being the largest party'.

"They've given up trying to be the government, they've given up trying to win a majority. They want to crawl into Downing Street on the coat tails of the SNP and put our country at risk. The British people will never have it."