David Cameron insisted on Monday he was "sure" there would be treaty change in Europe as he embarked on the start of a series of talks with EU leaders.
The prime minister is facing resistance from France and Germany over his plans to create fresh EU agreements ahead of his plan to stage an in/out referendum by 2017.
In his keynote speech on Britain's future in Europe earlier this year, Cameron argued a new settlement was needed before voters were asked if they wanted to end ties with Brussels and suggested some reforms would need treaty change.
Ahead of talks with French president Francois Hollande and German chancellor Angela Merkel, he told reporters three treaties had been put forward since he took the keys to No 10.
"So I'm sure there will be treaty change," he added.
"I'm absolutely convinced that there will be the need to reopen at some stage these treaties, not least to solve the problem of the eurozone.
"The eurozone in my view needs to have further treaty change, and just as eurozone countries will argue that it's necessary to have treaty change, I think it's perfectly legitimate to argue that non-eurozone countries might need to have treaty changes that suit them."
Cameron will make his first official visit to Madrid for bilateral talks with Spain's prime minister Mariano Rajoy this morning before travelling to Paris for a working dinner with Hollande. He will meet with Ms Merkel in Germany later this week.
The prime minister insisted he was confronting the Brussels problem in Britain and told newspapers in France, Germany, Italy, Spain and Poland reform was "not about cherry-picking" EU rules - something European leaders have claimed.
He said: "We are a major European power, a major European player. But do we think that the European Union has sometimes overreached itself with directives and interventions and interferences? Yes, it has. And that needs to change."
"There are some reforms I think we need to make. Already we're starting to make some of them."
He added: "The agenda of the speech is change that all of Europe can benefit from. It is a more competitive, open, flexible Europe for all countries of Europe. And the second thing is that - you know, this is not about cherry-picking, but to argue as some do that you can't have a flexible Europe is wrong. We have a flexible Europe.
"Britain is not in the single currency; neither are many other countries. Not all of us are members of Schengen. You know, some countries want to go ahead with the financial transaction tax. We don't. You know, so I think we can have a flexible Europe where we don't all have to do the same things in the same way at the same time. I think, as I say - as I argued in my speech that Europe will be more successful if it has the strength of flexibility rather than the weakness of inflexibility.
"I think the best outcome for Britain is our membership of a reformed European Union. But just as the two themes of my speech, if you like, are first that Europe needs reform, the second is that we need to recognise that consent for Britain's membership of the European Union, and all the ways that it's changed, has become wafer-thin in Britain. And politicians, if they do their job properly, have to recognise this fact rather than try and brush it under the carpet.
"The fact is that in British politics, the fact that parties and governments year after year promised referendums, didn't hold referendums when they could have done, that damaged consent for Britain's membership of the European Union and there's no good wishing that away. It exists; it's a fact. And the best thing to do when you have a problem is to confront that problem, deal with it.
"And to those people who say to me, 'Ah, but you're creating uncertainty', the greatest uncertainty would be to have this problem and to wish it didn't exist. Much better to have a plan for how we make changes to the European Union, how we make changes to Britain's membership, how we secure Britain's membership of a reformed European Union and we settle this issue. I have a plan."
The prime minister will also discuss his priorities for the G8 with all three European leaders, focusing on bolstering trade with the United States and doing more on tax compliance.