Liberal Democrat Cabinet minister Danny Alexander's Trident Alternatives Review is expected to include an array of options short of like-for-like replacement, including a proposal that the Royal Navy's fleet of nuclear-armed submarines could be cut from four to two.
The review of alternatives results from a compromise reached by Conservatives and Liberal Democrats in the 2010 coalition agreement which brought them into government together, and is likely to set a clear dividing line between the parties at the next general election.
Mr Alexander has said he hoped today's review would "open up a debate about the fact that there are different ways of approaching nuclear deterrents."
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