The future of Britain’s nuclear submarine fleet is set to become a major election issue this week amid concern that billions of pounds is being spent on a successor before parliament has approved an upgrade.

MPs will not vote on Trident’s replacement, the largest UK submarine project in a generation, until 2016. But a Ministry of Defence report, slipped out over Christmas, reveals that spending on the project’s “assessment phase” is to increase by a further £261m this year. Of this money, which will be drawn down from the project’s future budget, £206m will be spent on new facilities at the BAE Systems shipyard at Barrow-in-Furness.

On Tuesday the SNP and Plaid Cymru will force a debate on the Trident upgrade, which is expected to cost £20bn in total.

Angus Robertson, the SNP’s Westminster leader and defence spokesman, said: “When Labour MPs have voted with the Tories for another £30bn of austerity cuts, it is frankly scandalous that a further quarter of a billion pounds of taxpayers’ money is to be spent on Trident before parliamentarians even get a vote on whether or not to replace the system.

“In no other democracy, at a time of deep austerity and cuts, would over £3bn be spent on committing to such a massive project without consulting parliamentarians. Robbing the future budget of Trident only suggests they have already underestimated the eye-watering final cost.”

The MoD report, The UK’s future nuclear deterrent: 2014 update to Parliament, reveals that more than half a billion pounds – some £1.4m a day – was spent on the project in the last financial year alone. Much of this went on “long lead” items to build specialist parts for the new submarines, such as their propulsion and missile launching systems.

It is estimated that £1.24bn has been spent on the project so far, the same amount as chancellor George Osborne pledged to find in new money for the NHS. The MoD claims that its spending decisions will reduce the overall cost of the replacement programme by £42m.

But the report reveals that the total cost of the project’s assessment phase, at the end of which a decision on whether to proceed with Trident’s successor will be taken, will rise to £3.3bn, nearly £300m more than the MoD originally indicated.

The revelation has prompted fury from pro-disarmament parties, which could play a crucial role in any future coalition government.

They accuse the MoD of spending so much on the project that the next government will find it impossible to abandon.

The Greens joined the SNP and Plaid in accusing the government of seeking to bury a key issue for voters.

“The government intentionally pushed the review on our nuclear weapons capability back to 2016 to prevent it becoming a general election issue,” said Lesley Grahame, peace and disarmament spokeswoman for the Green party. “Every pound spent before the decision, before parliament gets to vote on the matter,, or a public debate is held, should be considered a very serious blow for democracy.”

Plaid’s Hywel Williams said: “We’re astonished that the government are, in this current climate of austerity, spending all this money, and front-loading it.”

The MoD and the Conservatives are believed to want the UK to have four submarines in any future deterrent fleet.

The Lib Dems suggest that the UK could get by with fewer submarines, which would not have to be on constant patrol as is the case now.

Labour leader Ed Miliband has indicated that he wants Trident replaced with a cheaper system. Many in his party would like it scrapped altogether. “This is a useless, hugely expensive virility symbol which will never be used,” said Labour MP Paul Flynn.

Peter Burt, director of the Nuclear Information Service said: “Major questions remain about the future of the Trident replacement programme after the next election. But the MoD seems determined to commit as much money to the project as possible before the election to try to force the hand of the next government.”

“The SNP debate this week in the House of Commons will be the only chance before the election in May to make Westminster think again about dumping this new generation of weapons of mass destruction in Scotland,” Robertson said. “Scottish Labour’s ‘branch office’ MPs must come clean about how they can even think about spending these huge amounts of money in such economically difficult times.”

An MoD spokeswoman said spending on the assessment phase for the successor submarine programme was approved by parliament in May 2011. “This phase is focused on progressing the design of the submarine and is expected to cost approximately £3.3bn, concluding with a ‘main gate’ decision in 2016,” she said. “No submarines are being built before then, and we expect the overall cost of the programme to remain within the initial 2006 white paper estimate.”