Public spending on a key Trident nuclear weapons system is due to rocket more than sixfold over six years, according to new figures from UK defence ministers.

The cost of taking part in a US programme to extend the lives of the D5 missiles that carry the nuclear warheads is officially forecast to rise from £5.5 million in 2010-11 to £37.5m in 2015-16.

This has been condemned as "dodgy accounting" and a huge waste by critics, who said the money would just help boost the profits of US arms companies. But the Ministry of Defence (MoD) insisted it was "excellent value for taxpayers".

The UK Government, under then-prime minister Tony Blair, agreed in 2006 to take part in a US programme, run by arms giant Lockheed Martin, to extend the lives of the D5 missiles by 14 years to 2042. This was despite the fact that the main decision to build replacement submarines to carry them is not due to be taken until 2016.

Until now, the yearly price of the D5 life-extension programme for UK taxpayers has not been known. But parliamentary answers to SNP defence spokesman Angus Robertson have revealed the escalating cost.

Defence ministers have said that planned annual expenditure on the programme will rise by £32m over six years (see table). They have refused to specify what spending is likely to be beyond 2015-16, as budgets have not yet been agreed.

"This answer shows another astonishing increase in costs connected with the unwanted Trident missile programme," said Robertson, who is also the SNP's leader in Westminster.

"An increase of nearly 600% in just six years, with ministers unprepared to even estimate the annual cost beyond 2016, is typical dodgy MoD accounting. That is a dramatic jump in costs by anybody's terms."

He told the Sunday Herald: "They even committed to spending this money on missiles before committing to building the submarines they are to be fired from. You couldn't make it up if you tried."

Robertson also pointed out that the money being spent on the UK's "sovereign" nuclear capability would go overseas. "Virtually all of the work will be conducted in the US, making a mockery of all the 'Project Fear' anti-independence campaigners' claims about no spending on sovereign capabilities outwith the UK."

Lockheed Martin at Sunnyvale in California was awarded a £520m ($848m) contract in 2007 to extend the life of D5 missiles. At the same time, another US military firm, Charles Stark Draper Laboratory in Cambridge, Massachusetts, was given nearly £200m ($318m) to help improve the guidance systems.

Robertson argued that the rising cost of the D5 programme raised the "terrifying question" of just how much Westminster's plan to replace the Trident system would cost in total.

The UK Government put the overall cost at £20 billion in 2006, but critics have argued it could be as much as £100bn.

John Ainslie, the co-ordinator of the Scottish Campaign for Nuclear Disarmament, described the D5 programme as "one of the worst examples of the misuse of taxpayers' money".

Ainslie said: "David Cameron is wasting millions of pounds on missiles that Britain will never own. We should spend this money employing an extra 1000 nurses, rather than subsidising American arms companies." The MoD, however, denied that the overall cost of prolonging the lives of D5 missiles had massively increased. It pointed out that the new figures were for individual years, not for the whole programme, the cost of which had been put at "some £250m" in 2006.

"Expenditure in the years 2013-14 and 2014-15 is greater than in 2010-11 because the Trident D5 life-extension programme has now entered its main procurement phase," said an MoD spokesman.

"It is entirely normal for the costs of an equipment programme to vary by year. Sharing the costs of the programme between the UK and US represents excellent value for the taxpayer."

The spokesman also discounted criticisms that Trident missiles were made in the US. "The UK nuclear deterrent is, and always has been, operationally independent," he said.