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.
More top news
Hundreds of operations will be cancelled and waiting lists will grow if the NHS is as busy as last winter, according to Labour analysis.
A 28-year-old man has been charged over the stabbing of a surgeon outside a mosque.
A quiet night to come across the UK.