The cost of replacing Trident has soared according to a government-ordered review which also sparked fears of job losses on the Clyde.

The price of building four replacement submarines for the ageing nuclear weapons system has risen from £25 billion to £31bn.

But defence chiefs have also set aside an extra £10bn in a contingency fund, meaning the final bill could be as high as £41bn.

Previous estimates have suggested that the overall cost of the decades-long Trident successor programme will eventually be up to £167bn.

But these have been based on the £25bn figure.

The Strategic Defence and Security Review (SDSR) also raised the spectre of job cuts in Scottish shipbuilding after ministers committed to build just eight new frigates there - not the expected 13.

Ministers also unveiled plans for nine new Boeing P-8 Maritime Patrol Aircraft, to be based at RAF Lossiemouth in Moray, and an additional two Offshore Patrol Vessels.

But the review also signalled big cuts ahead as the Conservative government seeks to slash £11bn from the defence budget.

A third of the land owned by the Ministry of Defence (MOD) is to be sold off, including potentially bases north of the Border, and one in three civilian staff will be sacked.

The SNP’s Westminster leader Angus Robertson rounded on David Cameron over the rising costs of Trident, asking the Prime Minister: “How expensive does it need to be for this Government to realise it’s a super-expensive vanity project that does not deter?”

Defence sources said that the increase was in part linked to inflation – as the earlier figure had been calculated in 2011 - and "more realistic" pricing.

There was also confusion as Downing Street suggested that MPs may be denied a vote on the final decision to replace Trident.

A No 10 source would only say that the government was committed to a "debate" on the future of the nuclear deterrent on the Clyde.

Mr Cameron previously promised MPs a say on the 'main gate' decision.

Laser he stepped in to reassure MPs they would get a vote “at the appropriate moment".

The SNP also accused Mr Cameron of "betraying" Scottish shipbuilders.

Brendan O’Hara MP, the SNP's defence spokesman, said: "It will not be forgotten that the UK Government promised 13 would be built – a cast iron guarantee. The UK government has today confirmed a 40 per cent cut in that pledge."

But the GMB trade union welcomed the SDSR and accused the Scottish Government of incorrect "spin", saying that the frigate announcement secured employment on the Clyde and warning of "political mischief".

The GMB and the other unions have arranged to meet BAE Systems in Glasgow next Thursday to discuss the outcome of the SDSR for the shipyards.

Gary Smith, acting secretary of GMB Scotland, also said an SNP-led non-binding vote on Trident today was the "Scottish political establishment indulging themselves".

Mr Cameron also hit back at the SNP saying that there was still "the opportunity" for 13 frigates and none of the ships would have been built if Scotland was independent.

Labour's sole Scottish MP Ian Murray, who is personally opposed to the replacement of Trident, has said he will abstain in today's vote, accusing the SNP of playing political games.

The debate is a one-line whip for Labour MPs, meaning they do not have to vote if they do not want to.

But a Labour source said that if MPs did vote they should the current party's current policy which, while under review, is to maintain a continuous-at-sea deterrent.

Overall the defence review outlined plans to spend £178bn over the next decade.

Mr Cameron has previously announced an extra £2.5bn, and more than 1,900 additional security and intelligence staff.

Ministers also committed to maintain the size of the Armed Forces.

Last month the chair of the Foreign Affairs Committee estimated that Trident replacement could rise to £167 billion, when maintenance of the system is costed in, based on the £25bn figure.

Kate Hudson, CND general secretary, said that ministers had “completely lost control” of the Trident budget.