Deputy Prime Minister Nick Clegg used a speech on Britain and Europe to say pro-Europeans need to take responsibility for the "negative reaction to all things continental".
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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."