"I'm sure there could be some disillusionment if this takes a long time, " Hansen said.

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As a result, "democratic consent for the EU in Britain is now wafer thin, " he said, and public disillusionment is at an all-time high.

CNN: Cameron promises referendum on Britain's place in Europe

Mr Cameron said "disillusionment" with the EU was "at an all time high" and "simply asking the British people to carry on accepting a European settlement over which they have had little choice" was likely to accelerate calls for the UK to leave.

BBC: David Cameron speech: UK and the EU