DAVID Cameron will shortly set out his full demands on reforming the European Union and has pledged to accelerate the pace of negotiations with his fellow heads of government amid claims that progress with them has, at best, been sluggish.

The move raises the prospect of the in-out referendum taking place in the autumn of 2016.

The Prime Minister will present the changes he is seeking in a letter to Donald Tusk, the European Council President, and which will be distributed to the other 27 EU leaders in early November.

Mr Cameron has promised a vote by the end of 2017 but is thought he wants to get the vote out of the way by the end of next year to avoid clashing with elections in France and Germany as well as the UK's six-month presidency of the EU.

However, Downing Street insisted that the timing would be driven by progress in the talks and not by any arbitrary timetable.

Meantime, London Mayor Boris Johnson ramped up pressure on the PM to secure significant reform, saying that the downside of Brexit was "not as big as many people think" and noting: "If we don't get the changes we want and we don't see progress, then you have got to be prepared to walk away."

The Tory MP suggested there could be "a new dispensation that involved Britain staying within the single market but being exempted from a lot of the other stuff".

Mr Cameron's agreement to provide more details of his ambitions followed signs of mounting impatience in Brussels over his failure to spell out exactly what reforms he is seeking.

As EU leaders gathered for the one-day summit in Brussels, Martin Schulz, the European Parliament President, called for "clarity" from the UK while Charles Michel, Belgium’s premier, said: "It is time for Mr Cameron to put his cards on the table."

The PM announced his initiative shortly after a working lunch with Jean-Claude Juncker, the European Commission President, who on Wednesday told MEPs that technical talks launched in June had not made "huge progress", adding: "It takes two to tango...our British friends have to dance".

Asked whether Mr Cameron’s announcement represented a decision that it was time for the UK Government to "dance", a British official responded: "This isn't Strictly; it's a negotiation."

Mr Cameron’s letter is expected to flesh out the demands in four key areas: welfare rules for migrant workers; the lifting of member states' commitment to "ever-closer union"; measures to protect non-euro states and to improve EU competitiveness as well as greater powers for national parliaments.

The PM insisted the renegotiation process he kicked off after winning the General Election was "going well" but that “the pace will now quicken” towards the December European Council.

He added: "I'm confident we can get a good deal for Britain, we can fix those things that need to be fixed and I'm confident this process is well under way and making good progress."