A "concrete proposal" for European Union reform will be tabled within weeks, Donald Tusk said in an apparent boost to David Cameron's hopes of securing a quick renegotiation deal.
The European Council president said it was "not easy but possible" that agreement could be reached between the other 27 member states at a summit next month.
That could open the door to the in/out referendum on Britain's continued membership of the bloc being held as early as June.
"I will table concrete proposal on #UKinEU in run-up to February #EUCO. I will work hard for deal in February, not easy but possible," Mr Tusk wrote on Twitter.
It came as Foreign Secretary Philip Hammond indicated they were looking at alternatives to a proposed benefits ban to stem the flow of EU migrant workers to Britain - the principal stumbling block to a deal.
The Prime Minister has made measures to cut migration one of the key demands in his renegotiation of Britain's relationship with Brussels, proposing a ban on migrants claiming in-work benefits until they have been in the UK for four years.
Mr Hammond told The Guardian that Britain was prepared to look at alternatives that would meet the aim of cutting migration and called the four-year proposal a "second-order approach".
Number 10 said Mr Cameron wanted a solution that would make a "concrete difference" to the numbers coming to the UK and had always been clear there were "a range of options on the table".
Cabinet minister David Mundell said there was a "strong argument" for holding the vote in June despite concerns about its close proximity to the Scottish Parliament elections in May.
The PM's spokeswoman refused to be drawn on whether June was Mr Cameron's preferred date but said: "We are going to be driven by substance not schedule. Then he will set out the timings for the referendum.
"But a deal in February would pave the way for getting on with this."
Delaying the referendum until after the autumn could lead to the vote being held after another summer dominated by the migrant crisis - something those in favour of remaining in the EU believe could deter voters from backing continued membership.