Dominic Cummings, the senior Tory official at the centre of party infighting, unexpectedly quit last night.
He resigned as Director of Strategy after just eight months to pursue "other things outside politics''.
His sudden departure will raise fresh questions over Iain Duncan Smith's judgement in appointing him to such a senior post at the age of 30. But it will delight supporters of the former party chairman David Davis, with whom Mr Cummings repeatedly clashed.
Mr Cummings had previously been campaign director at the anti-euro organisation Business for Sterling. He managed to enrage many senior Tories in June when he told The Independent that he believed the party should take a low profile in the No campaign in any future referendum on the single currency.
Mr Cummings said: "For many people just about the only thing less popular than the euro is the Tory party.''
He also made enemies among traditionalists within the shadow cabinet by arguing for the party to adopt "modernising'' policies.
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