David Cameron has given his strongest indication yet that he would call a referendum on EU membership if he was returned to power.
Speaking during his visit to Brazil, the Prime Minister said that there will be an opportunity after the general election for the British people to give "fresh consent" to UK membership of the EU.
Mr Cameron said he believes that remaining in the EU is in Britain's interests.
"I don't think it is in Britain's interests to leave the EU but I do think what it is increasingly becoming the time for is a new settlement between Britain and Europe, and I think that new settlement will require fresh consent, " he said during a round of TV interviews.
"In the next parliament, I think there will be opportunities for a fresh settlement and for new consent to that settlement. There is a reason why.
"The euro is a currency with 17 different countries. I think, increasingly, one currency will mean one economic policy.
"They are going to change and that will give us opportunities for changing our relationship with Europe.
"I argue for Britain's membership because I want to be able to say to countries like Brazil 'Come to Britain and you can sell to the 320 million consumers across Europe'.
"I argue for Britain's membership because I think it is in our interests. If I didn't think it was in our interests, I wouldn't argue for British membership."
Asked whether he would use the UK's EU opt-out on justice and policing, Mr Cameron told the BBC: "That has to be done by the end of the year.
"The opt-out is there, we will be exercising that opt-out. The key thing then is which of the array of things you can come out of do you actually think are good for Britain and you want to co-operate with European partners on, and that is the discussion we are having at the moment."
Shadow foreign secretary Douglas Alexander said: "Another foreign trip, and apparently yet another referendum policy from the Prime Minister.
"In past months he seemed to rule one out, now he seems to be hinting it might be in. It's no way to run a serious strategy on Europe.
"Labour are clear that this is not a decision we could or should sensibly make now. None of us can fully predict where Europe will be in a few months time, never mind in a few years time.
"Advancing and protecting Britain's interests in the coming discussions should be this Government's priority, not simply trying and failing to manage their restive backbenchers."