Revealed: MoD's new multi-million pound Trident deal with America

However, the UK parliament will not take the main decision on whether to renew Trident nuclear weapons, or how many submarines to have, until 2016, after next year's General Election.

This has angered leading SNP and Labour politicians, who say the UK Government is pre-empting a democratic decision.

The deal to equip UK ­submarines with 12 missile launch-tubes each is also at odds with the UK ­Government's repeated disarmament promise that it would only use eight.

Critics fear future governments could use the additional tubes to hugely expand the UK's nuclear firepower.

The UK Government is also facing allegations that it misled the public and the United Nations (UN) over the number of tubes, though this is denied.

The SNP's Westminster leader, Angus Robertson MP, warned such "startling" revelations should sound alarm bells. "It looks very much as if the MoD is planning to vastly increase capacity and capability to fire nuclear weapons of mass destruction," he declared.

The decision to spend £37m on the tubes "gave the lie" to the pretence the UK Government hadn't already decided to renew Trident, he said, and he pledged to raise the issue at Westminister as soon as possible.

Sarah Boyack MSP, one of the candidates to become Labour's Scottish leader, was "extremely concerned" the decision to renew Trident was being taken as read. "The MoD should not be pre-empting a decision of the UK parliament," she said.

"It's unacceptable that we're finding this out from US sources rather than the UK Government.

"Spending billions on acquiring a new weapons system would be going in the wrong direction."

The launch tubes, made in packs of four, are 45ft long and wide enough to stand up in. They each weigh more than 50 tonnes.

The US Navy said it has awarded a contract to General Dynamics Electric Boat for the construction of 17 launch tubes, 12 of which are for a UK submarine. The total value of the contract is £53m, with 70% of the cost met by the UK Government.

The plan is for General Dynamics to make 36 more tubes for three more UK submarines. The new contract "represents over five years of design and prototyping efforts between the US and UK governments and their respective industry partners," said the US Navy.

But in a 2010 defence review, the UK Government promised it would "reduce the number of operational launch tubes on the submarines from 12 to eight". This was part of a "commitment vigorously to pursue multilateral global disarmament", it said, and would help reduce costs by £3.2 billion over 10 years.

In October 2010, the UK's ­disarmament ambassador, John Duncan, told the UN the plan was to "configure the next generation of submarines accordingly with only eight operational missile tubes".

The US, however, has insisted on having 12 tubes per boat, and it now looks like its view has prevailed. The Ministry of Defence (MoD) has said new UK submarines will still have only eight "operational" tubes, but it has refused to say what will happen with the other four.

"Unfortunately we're not able to go into that level of detail at this stage," an MoD spokeswoman said. However, there is some speculation that if the tubes are not loaded with missiles, they could be filled with concrete to act as ballast.

John Ainslie, co-ordinator of the Scottish Campaign for Nuclear Disarmament, accused the UK Government of misleading the public and the UN by implying the number of tubes would be cut to eight. He pointed out that the first Trident submarines designed in the 1980s had four extra tubes so that more missiles could be loaded.

"History is now repeating itself," he said. "The MoD is leaving open the possibility that a future government could increase Britain's nuclear firepower by 50%."

Some of the launch tubes for the existing Trident missiles were made by Babcock in Renfrew.

"We are repeatedly told that Trident creates jobs in Scotland, but the reality is that the MoD has chosen to do a multi-million pound deal with General Dynamic in the US," he said.

"The Trident replacement programme is out of control, like a runaway train without a driver."

Richard Scott, a naval consultant editor for defence journal IHS Jane's, said the momentum for renewing Trident was gathering pace

"The submarine design and development programme is already shifting into a higher gear," he said.

"The MoD has already approved a series of investments which it sees as essential to the on-time delivery of the first submarine in 2028," he added. "The UK Government's commitment to buy the 12 missile tubes for the first successor boat represents one of a number of critical long-lead procurements to support delivery."

Ministers had committed almost £80m to the successor submarine project, Scott said. Money had gone towards development of a common missile compartment with the US, the modernisation of BAE Systems' submarine yard at Barrow-in-Furness, and helping Rolls-Royce develop a new reactor plant.

The MoD confirmed General Dynamics was making 12 launch tubes for a UK nuclear submarine but denied this pre-empted a parliamentary decision. "No submarines are being built before the 'Main Gate' decision in 2016," said an MoD spokeswoman.

"As with any major programme of this complexity, it is vital to order certain items that would delay the programme if we were to wait until Main Gate.

"The programme has been thoroughly reviewed by MoD, Treasury and Cabinet Office officials to ensure that we are minimising early spend."

A decision on whether to procure three or four submarines would be made in 2016, she said, and insisted the MoD had long made it clear the "baseline design" for new submarines was for 12 tubes.

The missile compartment designed with the US was to give sufficient space in the centre of the submarine for equipment, services and crew accommodation. "To move away from this common design would add cost and risk into the programme," she said.