In referendums the status quo option tends to do better that the initial polls suggest- this was seen in the referendum on electoral reform last year.
Currently a majority (or maybe a mere plurality at worse) of people favour leaving the European Union, but in a referendum the Europhile campaign will be free to scaremonger about the prospect of being "isolated in Europe" and taking a leap into the unknown. The worst the Eurosceptic camp can do is point at the present which cannot be made to seem unrealistically terrifying as an imagined future with less constraints of reality. Politically Ted Heath was quite clever to make the UK join the EEC and then let his successor have a referendum on whether to remain in- so the Europhile case became the status quo.
In a campaign the Europhiles will certainly close the gap on the Eurosceptics and may even overtake them to win the campaign. If that happens there will no constraint on UK governments who wish to surrender more power to Brussels as the issue would be "settled" for a generation.
Unless the support for withdrawing remains comfortably above 50% for a few years it would an enormous gamble to hold a referendum on the matter now.
Cameron’s Speech: Left Wing Commentariat Reacts
14 minutes ago