Deputy Prime Minister Nick Clegg said the "British people should have their say" if there is a "significant change" between the UK and Europe's relationship, during an interview with BBC Radio 4.
Mr Clegg said a referendum would take place if there was a new treaty.
Centrica's boss warns an EU referendum risks "total destabilisation" for businesses; the Next chief suggests that view is "scaremongering."
There will be sympathy, yes. But will they agree with him? No.
Conservative MPs said their "achievable" proposals to change Britain's role in Europe do not represent an ultimatum to the Prime Minister.