Prime Minister David Cameron is to publish full details of his demands for European Union reform, as he promised to "quicken the pace" of negotiations leading to an in/out referendum on UK membership.
The PM will set out the changes he is seeking in a letter to European Council President Donald Tusk to be distributed to the other 27 EU leaders in early November.
His decision to accelerate the progress of negotiations makes a referendum in 2016 more likely. The PM has promised a vote by the end of 2017, but is thought to want to get the vote out of the way by the end of next year, to avoid clashing with elections in France and Germany and the UK's six-month presidency of the EU. However, Downing Street insisted that the timing would be driven by progress in talks and not by an arbitrary deadline.
Meanwhile, 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 adding: "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.".
Mr Johnson 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 a one-day summit in the Belgian capital, European Parliament president Martin Schulz called for "clarity" from the UK, while Belgium's PM Charles Michel 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 European Commission president Jean-Claude Juncker, 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 the move represented a decision that it was time to "dance", a British official responded: "This isn't Strictly, it's a negotiation."
The letter is expected to flesh out demands Mr Cameron has already set out for changes to welfare rules for migrant workers, the lifting of member states' commitment to "ever-closer union", measures to protect non-euro states and improve EU competitiveness and greater powers for national parliaments.
Officials initially indicated that the letter to Mr Tusk may not be made public, sparking anger among eurosceptic MPs. The chair of the House of Commons European Scrutiny Committee Sir Bill Cash told PA it was "essential that people in this country and in our Parliament know what is being said to other member states on the future of our country".
When it was announced shortly afterwards that the letter would after all be made available to MPs and the public, Sir Bill said he was "extremely pleased".
Mr Cameron insisted that the renegotiation process he kicked off after winning the May general election was "going well" and the letter would take it into a new phase involving talks with other member states ahead of a "substantive discussion" at the European Council summit in December.
"The pace will now quicken and I'll be again setting out the four vital areas where we need change and laying down what those changes will be at the start of November," said the PM.
"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."
German Chancellor Angela Merkel said it was up to the UK to "clarify the substance of what it is envisaging" in the coming weeks, and warned the EU's principles of free movement and non-discrimination were "non-negotiable".
Following a meeting with Mr Cameorn, Mr Schulz said the European Parliament would be ready to discuss moves to improve competitiveness, but added: "Of course that can't be to the detriment of our social model. That's the belief of the Parliament."
With the referendum featuring only fleetingly in formal discussions in Brussels, the Vote Leave campaign said the PM had been "the invisible man" of the summit, while Ukip leader Nigel Farage said: "I am sure Merkel and Co are quaking in their boots as they wait for Cameron's renegotiation demands."
At a summit dominated by the migration crisis, Mr Cameron offered further UK help dealing with migrants arriving across the Mediterranean in countries like Greece and Italy. The UK will offer an extra 60 months' worth of assistance - involving for instance 10 officers for six months or five for a year - to EU border force Frontex's migrant-screening operation, and is ready to deploy 10 people to the European Asylum Support Office and offer additional support to tackle people smuggling routes in Greece, said officials.