The government today awarded contracts worth £350m to UK companies to design the next generation of nuclear submarines.
Most of the work on the Trident system will go to BAE Systems.
BAE said the contract will sustain the jobs of 1,000 workers at its site in Barrow-in-Furness, Cumbria.
The first Successor submarine is due to delivered in 2028, replacing the Vanguard Class vessels which currently carry the Tident.
"To deliver the assessment phase effectively, the MoD has signed a collaborative agreement with the three key suppliers in the UK submarine industry: BAE Systems Maritime Submarines, RollsRoyce and Babcock," the announcement said.
A decision on the final design and build will not be made until 2016, but the Ministry of Defence said detailed work has to take place now.
Defence Secretary Philip Hammond said: "This government is committed to maintaining a continuous submarine-based nuclear deterrent. The contracts announced today... symbolise an important step towards renewing our nation's nuclear deterrent into the 2060s.
"We have a world-class submarine-building industry in this country and this programme will help to sustain or create more than 1,900 jobs across the UK.
"By making the core equipment programme fully funded and affordable, we are able to confirm additional equipment projects which help safeguard our national security."
BAE Systems managing director John Hudson said: "The signing of this contract is a key step forward in our business strategy to deliver a seven-boat Astute programme followed by the replacement class for the Vanguard submarines.
"Not only does it help sustain the jobs of over 1,000 skilled employees currently working on the programme, it also provides the opportunity to grow our workforce by a further 280 in 2012."
First Sea Lord, Admiral Sir Mark Stanhope, said: "The Royal Navy has been operating continuous at sea deterrent patrols for more than 40 years and the Successor submarines will allow us to do so well into the future with cutting edge equipment."
The announcement could cause tensions within the coalition government. The coalition agreement stated that the Liberal Democrats would "make the case for alternatives" to fullscale replacement of the ageing Trident system, which would be scrutinised to ensure it represented value for money.
Conservatives are committed to a full renewal of the UK's fleet of four Vanguard-class submarines by 2028, but Lib Dem defence minister Nick Harvey has been conducting a review of possible cheaper alternatives.
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. Final decisions about ordering replacements do not have to be taken until 2016, after the next general election.