The price tag for replacing the Royal Navy's ageing fleet of Trident nuclear submarines could rise to as much as £40 billion, the Government has disclosed.
The cost hike was revealed in a Strategic Defence and Security Review (SDSR), in which Prime Minister David Cameron set out plans to spend £178 billion on buying and maintaining military equipment over the next decade.
But the new national security strategy also included big cuts to the Ministry of Defence's civilian workforce, which is set to be slashed by almost 30% to 41,000 over the next five years.
The MoD now estimates that acquiring four new submarines to carry the Trident deterrent will cost £31 billion over the course of the 20-year procurement programme - compared to a previous estimate of £25 billion.
And a further £10 billion "contingency" has been set aside to meet additional unexpected cost increases, reflecting the complexity of the project, with the first submarine due to enter service in the early 2030s, the SDSR said.
"The revised cost and schedule reflect the greater understanding we now have about the detailed design of the submarines and their manufacture," the report said.
The SDSR set out heavily-trailed plans for two new Army 5,000-strong "strike brigades" capable of deploying rapidly around the world, as well as an additional £12 billion of equipment funding. They will form part of a joint maritime, land and air expeditionary force totalling 50,000 by 2025, compared to the current commitment of 30,000.
Up to 10,000 troops will also be made available for swift deployment to support the police in dealing with terror attacks of the kind seen earlier this month in Paris.
In other measures, the MoD will acquire nine Boeing P8 maritime patrol aircraft to plug the gap left by the highly-criticised decision in the last review in 2010 to scrap a new generation of Nimrod aircraft.
There will be a 10-year extension to the operational lifespan of the RAF's Typhoon jets and upgrade work to give them ground attack capabilities - effectively adding two additional frontline squadrons.
The Typhoons - fitted with upgraded stealth radar equipment - will now see service through to 2040 to answer RAF fears over the dwindling size of its resources. The extended lifespan means there will be seven squadrons of around 12 aircraft.
The acquisition of the new F35 Joint Strike Fighter for the Royal Navy's two new aircraft carriers will be accelerated - with 24 aircraft available by 2023, rather than the previously planned eight.
The SAS and other special forces will get an extra £2 billion to improve their equipment, the RAF will double its number of drones, including unmanned craft which can "fly at the very edge of the earth's atmosphere and allow us to observe our adversaries for weeks on end".
An extra £1.9 billion will be spent on cyber security and 1,900 new personnel recruited to the intelligence agencies.
The number of new Type 26 frigates to be built on the Clyde was cut from the expected 13 to eight, though Mr Cameron said a new class of "light, flexible general purpose" ships would be developed to make up the numbers.
Meanwhile, a separate strategy for official development assistance set out plans to use the UK's aid budget to help bolster security, including by focusing half of the Department for International Development's budget on support for fragile and broken states.
The strategy included an increase to £1.3 billion for the Government's Conflict, Stability and Security Fund, as well as creating a £1.3 billion fund to drive global prosperity and good governance, investing £1.5 billion in scientific research into global problems like anti-microbial resistance, and £1 billion for work to counter infectious diseases.
In a statement to the House of Commons, Mr Cameron said the Government had taken "a clear decision to invest in our security and safeguard our prosperity".
The Prime Minister told MPs: "Our Armed Forces, police and security and intelligence services are the pride of our country. They are the finest in the world, and this Government will ensure they stay that way.
"Using our renewed economic strength, we will help them keep us safe for generations to come."
Labour leader Jeremy Corbyn told MPs that Britain "does need strong military and security forces to keep us safe", but said his party's own review would "recognise that security is about much more than defence".
Mr Corbyn was jeered by Tory MPs as he raised concerns about the security impact of issues like climate change, food security and cuts in neighbourhood policing.
Unions described the mooted cuts in numbers of civilian MoD staff - which stood at more than 121,000 as recently as 2000 - as "devastating".
Dai Hudd, deputy general secretary of the Prospect union, said: "Civilians were cut to the bone in 2010, so today's news of a 30% headcount reduction on top of that is devastating news, not just for jobs and skills but also for the UK's ability to deliver the capabilities it needs, on time and on budget."
The Campaign for Nuclear Disarmament said the total cost of Trident renewal could now reach a "staggering" £183 billion.
But the EEF manufacturers' organisation hailed the SDSR as a "welcome boost" for a "vital high-value, highly skilled" industrial sector, while BAE Systems chief executive Ian King said it provided "continuity and stability for our business".