David Cameron has revealed that the estimated cost of building four new nuclear submarines has soared by £6 billion.
The Ministry of Defence said the cost of renewing Trident was now £31 billion – up from £25 billion in 2011. The department is also putting aside another £10 billion in case the bill spirals further.
The costs were laid bare in the Strategic Defence and Security Review (SDSR) unveiled by the prime minister on Monday. He confirmed that MPs would get a vote on the renewal of Trident, despite the document simply pledging a "debate in parliament".
Cameron also announced that up to 10,000 troops could be deployed on the streets of Britain if terrorists launch a Paris-style attack. The military personnel will be "on standby" to help police with major incidents, along with bomb disposal specialists if needed.
Under the defence review, some £178 billion will be spent on defence over the next decade – including an extra £12 billion on equipment than was expected. But the MoD's civilian workforce will be slashed by almost 30% to 41,000 over the next five years – which unions warned would hit vital support for the armed forces.
Labour leader Jeremy Corbyn, a former chair of the Stop the War Coalition, was met with silence by Labour MPs as he responded to Cameron's statement in the House of Commons. Corbyn warned that cuts to the police service and to tax credits could undermine the work of the armed forces.
But by the end of his speech, his words were drowned out by the mutterings of MPs across the house. Cameron said: "The longer he went on, the less he had to say."
As part of the defence review, Cameron also announced:
* Two new 5,000-strong "strike brigades" will be formed from existing army forces by 2025, that can be deployed anywhere in the world at short notice.
* Nine new Boeing P8 maritime patrol aircraft will be bought to protect Britain from enemy warships and submarines.
* Britain will speed up the purchase of F35 Joint Strike Fighter aircraft, so that 24 will be operating from the navy's two new carriers by 2023 rather than the planned eight.
* A 10-year extension to the lifespan of the RAF's Typhoon jets will effectively create two extra squadrons.
* Eight Type 26 frigates will start to replace their Type 23 predecessors. But that's fewer than the 13 that navy chiefs wanted.
* An extra £2 billion will be spent on new equipment for the special forces, including the SAS.
Cameron said Trident was Britain's "ultimate insurance policy" and vowed to replace the four ballistic missile submarines.
The defence review states that Britain's nuclear deterrent "has existed for over 60 years to deter the most extreme threats to our national security and way of life".
It warns that as long as other states have nuclear arsenals, there is a risk they "might use their nuclear capability to threaten us, try to constrain our decision-making in a crisis or sponsor nuclear terrorism".
The Vanguard class of nuclear submarines will begin to leave service by the early 2030s and need to be replaced. The new submarines will last for 20 years after that, according to the MoD.
"Our latest estimate is that manufacturing the four successor submarines is likely to cost a total of £31 billion (including inflation over the lifetime of the programme), with the first submarine entering service in the early 2030s," the document states.
"We will also set a contingency of £10 billion. The revised cost and schedule reflect the greater understanding we now have about the detailed design of the submarines and their manufacture."
The replacement of Trident has long been a controversial issue in parliament and the revelation of soaring costs will no doubt fuel further protests. MPs are likely to get a vote on the renewal in the Commons next year.
But their first test will be on Tuesday when the Scottish National Party, which is opposed to Trident, will force a vote during an opposition day debate. The party hopes to expose the deep rift within the Labour party on the issue.
Leader Jeremy Corbyn has long campaigned against Trident's renewal, while shadow defence secretary Maria Eagle backs it. In September Corbyn said he would never use the nuclear deterrent if he was prime minister, even if it remained in place.