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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Mladic, now 75, was the military commander of Bosnian Serb forces in the 1990s, who fought against the Bosnian Croat and Bosniak armies.
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His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."