David Cameron will kick off his campaign to reform the EU in earnest by meeting Jean-Claude Juncker, the president of the European Commission, at Chequers on 25 May.
He is also due to hold talks soon after with French President François Hollande and German Chancellor Angela Merkel to gain support for reforms that will enable him to recommend that Britain stays in the EU.
Cameron, who was in Riga, Latvia, on Friday 22 May for a summit, insisted he would give voters a "proper choice" in a promised In/Out referendum on British membership before the end of 2017.
He said: "There will be ups and downs - you'll hear one day this is possible, the next day something else is impossible."
"But one thing throughout all of this will be constant and that is my determination to deliver for the British people a reform of the European Union so they get a proper choice in that referendum we hold - an In/Out referendum before the end of 2017.
"That will be constant. But there'll be lots of noise, lots of ups and downs along the way."
Brussels is to host an EU summit in June, but it was unclear whether Britain's demand for reforms, such as limiting the freedom of movement of people and cutting benefits for EU migrants, would be on the agenda.
Some eurosceptic Tories believe the Prime Minister is not going far enough with an opt-out of "ever closer union".
Daniel Hannan, the Conservative eurosceptic MEP, told the Radio 4 Today programme on Friday 22 May: "I think we're going for these quite minor technical changes - taking out the words 'ever closer union' or saying they'll disapply to Britain, changing the rules on benefits, which I think we can do by domestic legislation. I think there's an element of choreography about that.
"I don't think politicians get an automatic benefit of the doubt this time. I think there is a readiness to look at the small print and to be sceptical of big claims when people come back from these summits and declare victory."
Cameron seems able to rely on the support of Finland for his reforms.
Finnish Prime Minister Alexander Stubb was reported in The Times as saying: "Finland is very much in favour of the pro-reform agenda that David Cameron drives. We believe that there are issues that can be discussed with Britain. There is no point in putting up any barriers at this particular stage and we're all ears.
"I am hopeful that we will find a solution that settles British membership of the EU. Europe is, by definition, a compromise and I am sure we will find some kind of a compromise that is good for Britain and good for Europe."