David Cameron has insisted he needs an EU treaty change before he can sell a new deal on Europe to the British people, but confirmed he is looking to bring forward the date of his planned Brexit referendum.
In a sign of the urgency he attaches to the task, the prime minister is planning a series of bilateral meetings with EU leaders in the coming weeks ahead of a Brussels summit at the end of June.
Mr Cameron told his first cabinet meeting since his re-election last week that he had secured a mandate to change Britain's relations with Europe, but he acknowledged the talks would be "tough".
Downing Street said Mr Cameron was sticking to his deadline for holding an in-out referendum by the end of 2017, but that he would accelerate the timetable if it was possible.
"If we can do it earlier we will," Mr Cameron's spokesman said. "There has been no change in the position. The prime minister has set it out on a number of occasions."
Legislation enabling the referendum could be on the statute book by early 2016, but British negotiations with 27 other capitals and the European Commission could be complex and time consuming.
The spokesman added: "He wants treaty change. All the advice that he has had is that treaty change is required, for example in terms of some of the changes that we want to see in welfare."
However, much depends on what exactly Mr Cameron means by "treaty change", especially as other EU member states have ruled out any significant changes to the bloc's legal texts.
There is no appetite for any change that would require any member state to hold a ratification referendum, but Brussels lawyers are adept at using protocols, declarations and political agreements to solve such problems.
Mr Cameron said in an interview with the Financial Times in March he had been advised he would need treaty change to allow Britain to impose a four-year qualification period for migrant workers before they could claim benefits.
But he admitted that legal advice in the EU was "a strange beast" and that Brussels lawyers were skilled at finding their way round apparently awkward treaty issues.
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>Another option being considered by British officials is to set out a protocol with legally binding "postdated" treaty changes, to be incorporated at a later date when the EU next needs to agree changes to its rule book.Asked whether Mr Cameron hoped to start the British renegotiation formally at the June EU summit, his spokesman said: "The prime minister will bring it up at the European Council, clearly it is something he wants to get under way."
European leaders are waiting to hear the details of what Mr Cameron wants to renegotiate. They say that the broad principles outlined so far could encompass a range of different options, some of which are more workable than others.
They want to know, for example, whether Mr Cameron intends to prevent EU immigrants from claiming both in-work and out-of-work benefits for four years.
While some in Brussels believe it would be possible for the UK to ban out-of-work benefits for four years, they believe doing the same for tax credits and other payments to those with jobs would fall foul of European laws on non-discrimination.
One European official said: "The ball is in Cameron's court. We will want to see proper detail on what he is asking for, not just a broad outline."
British ministers say the broad outline of the UK position is already in the public domain. It includes curbs on migration, safeguards for the UK in the single market against any eurozone protectionism and an exclusion from the EU's "ever closer union" provisions.
Mr Cameron also wants national parliaments to be able to work together to block EU legislation, although this could be legally more problematic, and a commitment to return some powers to member states.
Additional reporting by Kiran Stacey
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