Prime Minister David Cameron has told the Commons he would "rule nothing out" if he can't secure an agreement on the EU reform proposals.
He insisted Britain would be best off remaining in a reformed Europe.
But he added: "Let me say again, if we can't secure these changes I rule nothing out."
Mr Cameron also said he will never say the EU is "fixed", even if the changes are secured.
Mr Cameron said: "There'll be many things that remain to be reformed and Britain would continue to lead the way."
David Cameron is expected to warn that voting for a "Brexit" would result in refugee camps springing up across the south-east of England.
If voters chose to stay - whenever the referendum comes - the deal upon which that decision was reached may not be the final word.
As his tour of Europe gets underway, David Cameron faces a fight to stop his proposed EU deal being watered down further.