Some EU countries have made clear they have no appetite to reopen the bloc`s treaties to suit Britain, which wants to alter them so it can restrict and delay EU migrants` access to its welfare system.
But London has long asserted that the treaties would need to be overhauled anyway as part of an inevitable drive to further integrate the euro zone.
Two-speed Europe
In a potential setback for Cameron, a Franco-German paper shows the two countries have agreed plans to strengthen cooperation among the 19 countries using the euro currency, without changing existing treaties.
British Foreign Secretary Philip Hammond said he expected some EU states initially to adopt a hardline negotiating position.
"What matters is getting it right rather than doing it quickly," Hammond said of the renegotiation last week. "We`re certainly not going to trade substantive reform just for getting it done quickly."
German Vice Chancellor Sigmar Gabriel said last week that Europe may have to move at different speeds in the future, with Germany and France taking the lead and leaving others to follow later.
"We need in Europe more courage for different speeds in co-operation," Gabriel wrote in an opinion piece published in Germany tabloid Bild on Friday (29 May).
"Some - especially France and Germany - need to lead the way, for example in energy policy or in the co-operation of economic and finance policy. Others can follow when they are ready," he wrote.
"Not everyone needs to do everything. But we need deeper co-operation under the roof of the European Union."
Gabriel said Europe needed to reform to become more social and fair, and called for more investment and growth to create jobs, as well as a fair distribution of migrants from North Africa.
Cameron`s government introduced a law into parliament on Thursday to guarantee the EU referendum will be held by the end of 2017. It also disclosed the question voters will be asked, making it "Yes" to stay in, "No" to leave.
"My priority is to reform the European Union to make it more competitive and address the concerns of the British people about our membership. The status quo is not good enough," said Cameron, when he visited French President Francois Hollande last week as part of a whirlwind European tour to try to drum up support for European Union reform.
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