Britain cannot get access to the European single market without accepting the free movement of workers, the European Commission president has said.

Jean-Claude Juncker insisted there would be no compromise to allow the UK access to the free trade zone while also being able to fully control immigration of European Union citizens after Brexit.

The conundrum has become central to the 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 Theresa May as essential to properly implement the referendum result.

The Prime Minister slapped down Brexit Secretary David 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.

With Britain's withdrawal process clouded in confusion and no clear signal on whether the country intends to stay in the single market or not, Europe's 27 other leaders gathered for an informal summit in the Slovakian capital, Bratislava, without the Prime Minister.

Asked about Brexit at a press conference, Mr Juncker said: "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."

Meanwhile, European Council president Donald Tusk revealed that Mrs May could be targeting early 2017 to trigger Article 50 - the process of leaving the EU.

Formal negotiations between Britain and the EU cannot begin until she does.

Speaking about his recent visit to the UK, Mr Tusk said: "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."

The leaders of Germany and France are keen to present a united post-Brexit vote front for the EU as both countries hold crunch national elections next year.

The refugee crisis, counter-terrorism, and tax regulation, are major issues at the summit.

Uncertainty over the pace, and scope, of Brexit was highlighted by former European Council president Herman Van Rompuy who said detailed withdrawal negotiations may not get under way until the end of 2017.

Mr Van Rompuy described the outcome of the EU referendum as a "political amputation".