Liberal Democrat party leader Nick Clegg said he was "shocked and saddened" by Chris Huhne's guilty plea but said he had "taken the right decision in resigning as an MP".
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Extracts released by 10 Downing Street suggest Mrs May will say the UK will not settle for a "half-in, half-out" policy.
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In her Brexit speech on Tuesday, the Prime Minister is set to urge the EU to forge a fair trade deal or risk commercial war.