The estimated cost of replacing the submarines which carry Britain's nuclear weapons has shot up by 6 billion pounds (8.5 billion euros, $9.0 billion), the government revealed Monday.

The latest estimate of 31 billion pounds features in the Strategic Defence and Security Review (SDSR), the government's five-year strategy for defence.

That compares to a previous defence ministry estimate of 25 billion pounds which was still being quoted earlier this year.

On top of the 31 billion pounds, the SDSR also factors in an extra 10 billion pounds of "contingency" funding for the project.

Britain currently has four submarines in its ageing Trident fleet, at least one of which is on patrol somewhere in the world 24 hours a day.

The government wants to replace those with four so-called Successor submarines, the first of which would enter service in the early 2030s.

Ministers are due to take the final decision on replacing them in 2016 but the issue is intensely controversial.

The leader of the main opposition Labour party, Jeremy Corbyn, is opposed to nuclear weapons though some of his MPs support them.

The pro-independence Scottish National Party is also fiercely against Trident and is leading a House of Commons debate on the issue Tuesday.

The nuclear submarines are housed at Faslane naval base west of Glasgow.

Most MPs had long assumed they would be given a vote on whether to replace Trident but Downing Street indicated Monday that that may not be the case.

"We are committing at this stage to a debate," Cameron's spokeswoman told reporters.

"We are not putting a time on it. We are not ruling out that there will be a vote."

The Trident announcement comes as the SDSR confirmed there would be major budget cuts elsewhere in the Ministry of Defence, including laying off 30 percent of its civilian staff.