THE US nuclear-armed missile that the Westminster government is hoping will replace Trident may not actually fit into British submarines, creating a "major headache" for UK weapons designers.

The Sunday Herald has seen evidence that US designers are contemplating new missiles too big to slot into the tubes that house Trident's current D5 missiles.

Tenders to bid for a test-bed for future underwater-launched nuclear missiles issued by the US navy last month specify a missile diameter of up to 120 inches. The diameter of Trident's D5 missile tubes is 87 inches.

The former prime minister, Tony Blair, won the support of the House of Commons in March this year for his controversial decision to renew Britain's nuclear weapons by replacing the four Trident submarines, currently stationed at Faslane on the Clyde.

Blair's plan - apparently endorsed by his successor, Gordon Brown - is to start arming the new submarines with the existing Trident missiles but then to replace them with new missiles being designed by the US.

That is why Blair exchanged formal letters with US president George W Bush on December 7, 2006. "The United Kingdom wishes to ensure that any successor to the D5 system is compatible with, or is capable of being made compatible with, the launch system for the D5 missile, which we will in the meantime be installing into our new submarines," Blair wrote.

Bush replied by inviting the UK to take part in the D5 replacement programme or to discuss extending the life of the missiles. "In this respect, any successor to the D5 system should be compatible with, or be capable of being made compatible with, the launch system for the D5 missile," he wrote.

Earlier this month defence secretary Des Browne confirmed that UK and US officials had met three times since March to consider missile designs. "Concept studies for the development of a new underwater-launched missile system have been discussed by officials at these meetings," he said.

But critics are now saying that Bush appears to be reneging on his promise to make sure any new missiles would be compatible with existing D5 launch systems. In November the US navy issued a notice inviting companies to bid for a new test-bed for "development testing of underwater-launched missile systems".

The notice said the test-bed should be able to support missiles up to 120 inches in diameter and 200,000lbs in weight, although Trident missile tubes have a diameter of 87 inches and the missiles weigh 130,000lbs. This was because "concepts for future submarines may have missile tubes larger than 87 inches in diameter", the notice said.

John Ainslie, co-ordinator for Scottish CND, said this would going to give the engineers in Barrow responsible for designing Britain's new submarines a big problem. "Common sense would suggest that if you are designing a submarine you must know the dimensions of the missiles it will carry," he said.

"But common sense has no place in the government's plan to build a new nuclear weapon system. The rushed programme to replace Trident could set a new benchmark for ineptitude at the Ministry of Defence MoD.

"Gordon Brown should call a halt to this absurd waste of taxpayers' money."

Ainslie argued it was not credible to arm the new submarines with Trident D5 missiles throughout their life because the US would cease maintaining them. The US navy is planning to withdraw Trident completely by 2042, but the UK wants to operate its new submarines until at least 2055.

He also pointed out that when the UK first bought Trident missiles in the early 1980s, it had to change its order to fit US timescales.

"They will be keen to purchase an underwater-launched missile system. But designing a submarine for an unknown missile will be a nightmare."

A spokeswoman for the MoD said: "We are satisfied the exchange of letters between the previous prime minister and the US president provide us with the necessary assurances that any US successor to the D5 missile should be compatible, or can be made compatible, with the launch system to be installed in our new submarines."