It is understood Denis MacShane formally applied for the post of Crown Steward and Bailiff of the Chiltern Hundreds tonight - the traditional way of resigning from the Commons as there is no way for MPs to resign.
Chancellor George Osborne is expected to confirm his appointment tomorrow, at which point he will cease being an MP.
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The US first lady travelled without her husband to support the Invictus Games, which were founded by the Prince in 2014.