Replacing the Royal Navy's ageing fleet of submarines that carry Britain's Trident nuclear deterrent could cost up to £40 billion, the Government has disclosed.
The Strategic Defence and Security Review (SDSR) unveiled by Prime Minister David Cameron in the Commons also includes big cuts to the MoD's civilian workforce.
The Ministry of Defence now estimates that acquiring four new submarines will cost £31 billion over the course of the 20 year procurement programme - compared to a previous estimate of £25 billion - according to the Strategic Defence and Security Review (SDSR).
And a further £10 billion "contingency" has been set aside 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.
However the MoD's civilian workforce will be reduced by almost 30% to 41,000 over the next five years.
There will also be a new "pay model" for armed forces personnel - with an offer to new recruits that is intended to better meet their expectations and "targets resources on the people we need the most".
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 will now see service through to 2040 to answer RAF fears over the dwindling size of its resources - with the extended lifespan meaning there will be seven squadrons of around 12 aircraft.
As well as being enhanced to add ground attack capability to their aerial combat role, they will be fitted with upgraded stealth radar equipment.
The acquisition of the new F35 Joint Strike Fighter for the Royal Navy's two new aircraft carriers will be accelerated - with 24 aircraft to 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, an extra £1.9 billion will be spent on cyber security and 1,900 new personnel recruited to the intelligence agencies.
Speaking at RAF Northolt ahead of his Commons statement, the Prime Minister said: "This is showing that there is no economic security without national security and vice versa.
"We have now got a stronger economy and we can choose, rightly, to invest more in our national security - more ships, more planes, a bigger navy, a bigger RAF, a better equipped army, better in terms of fighting cyber attacks and fighting terrorism.
"Britain is the only major country anywhere in the world that both meets its Nato spending targets and meets its aid commitments.
"We are an engaged nation, not for reasons of national vanity but for reasons of clear-eyed self interest. What goes on in the world matters to the United Kingdom and so we should be helping to shape it.
"And with today's announcement we can do just that."