Former cabinet Minister Michael Portillo said it is "very important" to opt out of Europe if we have a referendum.
Speaking to Daybreak he said: "If the country votes to stay in that would be taken as a green light as full integration, we then over a period of time would go into the Euro, go into the integration process that is necessary to keep the Euro going".
Cameron has pledged to back a Conservative MP's EU referendum bill, as Cable labelled the move potentially 'damaging' for the country.
114 Conservative backbenchers have voted in favour of a eurosceptic motion to amend the Queen's Speech in defiance of the Prime Minister.
David Cameron has denied he has buckled under pressure from his backbench MPs over the EU debate.