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.
Liberal Democrat 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.
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