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
Nobel-prize-winning schoolgirl Malala Yousafzai has said she is a proud Brummie, as a new portrait of her was unveiled in her adopted home.
The Filipino girl group lost out to Lauren Murray in the sing off after receiving the lowest number of votes.
Shoppers in the UK are expected to spend almost £1bn online tomorrow for Cyber Monday as a weekend of discounts comes to an end.