European Council president Donald Tusk said the Prime Minister told him during their meeting at Downing Street last week that she would be ready to trigger Article 50 to begin Brexit by February 2017.
Formal negotiations between the UK and the EU cannot begin until she starts the two year process, which Brexit Secretary David Davis has insisted will be triggered without a parliamentary vote.
Speaking about his meeting with the PM, Mr Tusk told a summit press conference: “Prime Minister May was very open and honest with me.
“She declared that it’s almost impossible to trigger Article 50 this year but it’s quite likely that they will be ready maybe in January maybe in February next year.”
Meanwhile, Defence Secretary Sir Michael Fallon vowed to block any proposals for an EU army while Britain remains a member of the union in a move likely to anger European leaders.
But Sir Michael said the UK would veto plans for any EU army that may rival Nato.
“That is not going to happen,” he told The Times. “We are full members of the EU and we will go on resisting any attempt to set up a rival to Nato.
“We have always been concerned about unnecessarily duplicating what we already have in Nato.”
In Bratislava, Mr Juncker insisted Britain cannot get access to the European single market without accepting the free movement of workers.
The conundrum has become central to the Brexit debate and has caused friction within the Government as ministers weigh up how to maintain the benefits of the trading bloc while regaining full control of the UK’s borders - a goal deemed by Mrs May as essential to properly implement the referendum result.
The PM slapped down Mr Davis for saying it was “very improbable” that the UK could be in the single market and end free movement, and then insisted the Government would not offer a “running commentary” on negotiations.
Commenting on the issue, Mr Juncker told the press conference: “There’s a clear interlink as we made clear since the very beginning between the access to the internal market and the basic principles of the internal market, mainly the one of the freedom of movement of workers.
“We are sticking to that position and this is not a game between prime ministers leaving and prime ministers remaining, this is about people in Europe.
“It’s about the rights of ordinary people and workers, of those living in Europe, and so I can’t see any possibility of compromising on that very issue.”