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In promising to renegotiate Britain's relationship and promising the referendum, it is Mr Cameron's presumption that Europe would have to look again at its treaties and it would be at that point that at which he would jump in.
Even the UK's closest ally in Europe, the Netherlands, did not hold out the prospect of that tonight, the Dutch Prime Minister says they would do everything they can to fix the Eurozone within existing treaties.
But Downing Street insist it is inevitable. But in terms of Mr Cameron's back benchers and in terms of voters who might have been tempted by UKIP in the Eastleigh by-election - we will have to see.