THE real cost of maintaining and replacing the Trident nuclear weapons system over the next 50 years could top £100 billion - five times higher than the prime minister, Tony Blair, has said.

A new analysis of projected spending based on official figures suggests that the cost of buying and operating a successor to Trident will be around £70bn. Added to that, there is the £30bn it will cost to keep the existing warheads in service until 2023.

This contrasts with the £15bn-20bn highlighted by Blair and other ministers as the cost of buying a replacement to Trident. "Tony Blair is trying to persuade parliament to sign up to his nuclear insurance policy without revealing its true cost," alleged John Ainslie, the co-ordinator of the Scottish Campaign for Nuclear Disarmament (CND).

"The government is trying to con the taxpayer into spending over £100bn on weapons of mass destruction that we don't need and don't want. It will be our schools and hospitals that will suffer if this plan is approved."

In the past, ministers have said that maintaining Trident absorbed no more than 3% of the total defence budget. But recently, they have increased this figure to 5%-6%.

In evidence to the House of Commons Defence Committee last week, the defence secretary Des Browne admitted that the cost estimates had been revised. "We went through an exercise recently to make sure that we were identifying as accurately as we could the costs that are associated with our nuclear weapons systems," he said.

He accepted that it was "perfectly legitimate" to assume that Trident would continue to absorb 5%-6% of the defence budget. On that basis, calculations suggest, the total cost of maintaining and replacing Britain's nuclear weapons between now and 2054 will be between £90bn and £110bn.

The Ministry of Defence did not reject these figures, and reiterated that costs were expected to remain at 5% or 6% of the defence budget. "To try to extrapolate running costs for the whole 50-year period from that is inevitably highly speculative," an MoD spokesman said, adding that the cost was less than 0.1% of gross domestic product, and a "price worth paying for continued security".

Blair announced his intention to replace Trident in December, and MPs are expected to vote on the issue in the Commons next month.

Tomorrow the anti-nuclear group Greenpeace is promising to "name and shame" 27 Scottish Labour MPs who are either backing Trident or have refused to state their position.

A billboard bearing all the names will spend the next 10 days touring Scotland alerting voters to the stance being taken by their MPs. As part of the campaign, the Greenpeace ship Arctic Sunrise will be open to visitors in Leith docks today, before sailing to Greenock and then London.

"Three-quarters of Scottish people oppose spending billions of pounds on new nuclear weapons," said Greenpeace's disarmament campaigner, Louise Edge. "Yet the majority of Scottish Labour MPs are either not saying how they will vote, or have made it clear they will vote to replace Trident."

A survey of Scottish MPs' voting intentions on Trident by the new umbrella group, Scotland's For Peace, lists 12 Labour MPs, including 10 ministers, as "won't oppose" and 16 as "not known".

It says 11 Labour MPs will vote against Trident replacement, including the former minister Gavin Strang.

All 12 LibDem MPs in Scotland are listed as "not known", while the six Scottish Nationalist MPs will vote against.

Scotland's single Conservative MP, David Mundell, is likely to back Trident.