The Government may not be able to afford to renew its Trident nuclear deterrent because it will face a series of large bills for military projects at the same time, a former defence minister has said.
Sir Nick Harvey said the Ministry of Defence was scheduled to pay for the Joint Strike Fighters, new Type 26 frigates and unmanned aircraft when Trident was coming up for renewal.
The Liberal Democrat MP for North Devon, who lost his Government job in last month's reshuffle, urged David Cameron to consider whether it was necessary to spend millions renewing the missile system.
Speaking during Prime Minister's Questions, he said: "In turning to the Trident issue, have you looked at the severe cost pressures facing defence at the very moment Trident has to be paid for - Joint Strike Fighter airplanes, Type 26 frigates, unmanned aircraft, Army vehicles? All need paying for at much the same time.
"Now this has to come out of the defence budget and austerity is with us for some time yet. Will you keep an open mind as how to exactly replace our nuclear deterrent?"
But Mr Cameron insisted that the Government had planned for the expenditure.
He said: "The list of things that you go through are all programmes that are fully funded and are going to be properly invested in because this Government, as you well know as you played a major role in it, has sorted out the defence budget.
"What I would say about the new nuclear deterrent is that I don't believe we would save money by adopting some alternative nuclear deterrent posture. Also, if you are going to have a nuclear deterrent, I think it makes sense to have something that is credible and believable, otherwise there is no point in having one at all."
Earlier, Tory MP Julian Lewis (New Forest East) said replacing Trident, which sees submarines armed with nuclear missiles constantly at sea, with a part-time deterrent could be "dangerously destabilising".
Mr Cameron said the Government was already working with defence manufacturer BAE to design a new Vanguard class submarine. Maintaining a "continuous at-sea deterrent" is a key part of the UK's "credible" defence strategy, the Prime Minister added.