Defence Secretary Philip Hammond insists the final decision on Trident had not been made as he confirmed a £1 billion contract for reactors to power the next generation of Britain's nuclear submarines.
Replacing Britain's nuclear deterrent has proved a flashpoint for the coalition with Liberal Democrats opposed to "like-for-like" replacement and Conservatives committed to a full renewal of the UK's fleet of four Vanguard-class submarines by 2028.
Mr Hammond will formally announce the contract for two reactor cores, one of which will be used for the seventh Astute Class attack submarine and one for the first of the next generation nuclear deterrent submarines.
The funding will also be used for an 11-year refit of the Rolls-Royce five-year-old plant at Raynesway, Derby, which will carry out the highly specialised work. The deal will directly create 300 jobs.
Mr Hammond said the final "main gate" decision on Trident renewal will still not be made until 2016, but long lead times mean the reactor contracts needed to be signed now.
He told BBC1's Sunday Politics: "The actual decision to go ahead and build them won't have to be taken until 2016 and what we are doing at the moment is ordering the things that have to be ordered in order to give us that option.
"We have already done a review of options and value for money of the Trident programme and that concluded that replacing the Vanguard submarines and continuing with Trident was the best-value solution to maintaining a nuclear deterrent. But the Liberal Democrats wanted to have another look at some emerging technologies and Nick Harvey, the Armed Forces minister, is leading a review to look at whether there are any."
The first of the four Vanguards had been due to leave service in 2022, but the Government extended the vessels' lives as part of the 2010 Defence and Security Review.
Last month Mr Hammond awarded contracts worth £350 million to UK companies to design the next generation submarines.
Scottish nationalists, who are opposed to Trident, criticised the £1 billion contract. Angus Robertson, SNP defence spokesman, said: "People in Scotland do not want Trident. Church leaders, the Scottish Trades Union Council, The Scottish Government and the Scotland's Parliament are all against weapons of mass destruction being in our waters.
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