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The government has insisted a final decision has not yet been taken on replacing the Trident nuclear deterrent, despite a £1.1bn contract for reactor cores being awarded.

The SNP's Angus Robertson said the move was "a democratic affront" and an "obscene waste of money".

Green MP Caroline Lucas called the contract an "insidious attempt to pre-empt Parliament's decision" on Trident.

But Defence Secretary Philip Hammond said it was "good news" for the UK.

Britain's nuclear weapons system is currently made up of four Royal Navy Vanguard submarines, based at the Faslane naval base on the Clyde, which can deploy Trident ballistic missiles carrying nuclear warheads.

The Ministry of Defence says the £1.1bn investment secures 300 jobs at Rolls-Royce and will also fund an 11-year refurbishment of its plant at Raynesway in Derby.

But the government has said that a final decision will not be taken on Trident until 2016 - after the next general election.

Responding to an urgent question from Mr Robertson in the Commons on Monday afternoon, Mr Hammond said it was necessary to commission the work on "long-lead items" ahead of that deadline.

He said the Rolls-Royce reactors would power the Astute class attack submarines and the replacement for the Trident missile-armed Vanguard class, subject to final approval.

But the SNP's Westminster leader said the announcement "paves the way for Trident renewal... in the face of opposition in Scotland".

He said the majority of MPs and MSPs were against it, as were trade unions and religious leaders.

"The Westminster government is aware of these objections but it is ploughing on regardless. And then, at the end, it plans to dump this next generation of weapons of mass destruction on the Clyde.

"This is a democratic affront and it is an obscene waste of money."

Spending the money on infrastructure projects instead would create 10,000 jobs directly and a further 4,900 indirectly in related industries, Mr Robertson said.

But the defence secretary accused him of "hyperbole" and said the announcement was "nothing to do with weapons".

He added that 6,000 jobs depended on the Scottish naval bases.

The Ministry of Defence has already set aside £3bn to begin work on the new submarines to replace the four Vanguard class boats.

Ms Lucas said the taxpayer was "having to stump up for technology that may not even be needed, while our public services take the hit from austerity".

"This insidious attempt to pre-empt Parliament's decision seriously undermines our democratic system, and sends out a worrying signal to the rest of the world about the UK's commitment to nuclear disarmament," she added.

The Liberal Democrats are currently conducting a review into possible alternatives to replacing Trident.

Mr Hammond said its findings would be taken into account when the "main-gate" decision on its successor was taken in 2016.

Labour said the development of the new reactors must go ahead "whether or not there was a final decision on Trident" because they were crucial to the UK's wider defence capability.