European leaders look set to give the green light to six months of behind-the-scenes talks on Britain's demands for changes to the terms of its EU membership.
David Cameron's plans for a renegotiation followed by an in/out referendum are on the formal agenda for the first time at the summit of the 28 leaders in Brussels, in what the Prime Minister said was "a significant milestone".
A leaked draft of the communique to be released after the conclusion of the two-day European Council meeting on Friday suggests that leaders are expected to agree to discussions taking place at official level, before they "revert" to the issue at the next scheduled summit in December.
Mr Cameron was this morning completing his whirlwind diplomatic effort to speak to all 27 fellow leaders about his plans before the summit opens.
After arriving in Brussels from Berlin this morning, he met the prime ministers of Portugal and Bulgaria. And he was later due to meet the president of Lithuania and chancellor of Austria, as well as speaking by phone with the prime minister of Croatia and Greek PM Alexis Tsipras - although Downing Street acknowledged that Mr Tsipras may be unable to break off from discussions about his country's debt crisis.
The increasingly desperate efforts to resolve Greece's economic woes and allow the country to remain in the euro were set to dominate the summit, along with discussions about the Mediterranean migrant crisis, squeezing the time and attention available to discuss Mr Cameron's concerns over the terms of the UK's membership.
Britain's demands for reform will not be discussed around the negotiating table, but leaders will have "an opportunity to listen" to Mr Cameron set out his position over a working dinner.
The summit comes a day after the Queen highlighted the need for European unity in a speech during her state visit to Germany.
Speaking at a banquet attended by Mr Cameron and German Chancellor Angela Merkel in Berlin, the Queen said it was necessary to "work hard to maintain the benefits of the post-war world", adding: "We know that division in Europe is dangerous and that we must guard against it in the West as well as in the East of our continent."
Asked whether Her Majesty took advice from Downing Street in the usual way about the content of her speech, the Prime Minister's official spokeswoman said: "You would expect us to follow the normal process as we would for speeches made by the Queen at banquets around the world."
The Prime Minister wants "technical" talks to begin at official level before the summer to address the UK's concerns on issues including a British opt-out from the requirement for "ever-closer union" in Europe, restrictions on benefits for EU migrants, measures to improve economic competitiveness, a greater role for national parliaments in EU decision-making and protection for the interests of EU members, like the UK, which do not use the euro.
He believes that the changes he is seeking will require change to the EU's treaties - something many member states are thought to be wary of.
Arriving at the summit, Mr Cameron said: "Today marks a significant milestone in the process of saying that it is right for Britain to have this renegotiation and this referendum to address the concerns that the British people have about Europe and to make sure that the British people have the final say about whether we stay in a reformed Europe or leave. That's what today is about.
"Today we are also discussing this vitally important issue of European migration, where we need a comprehensive and co-ordinated approach, and Britain will play its role. Already, it is the Royal Navy picking up people in the Mediterranean.
"British aid is doing a huge amount to stabilise African countries - and we can do more - and we are bringing our expertise to bear in terms of cracking down on those criminal gangs that are trafficking people across Africa and into Europe.
"I look forward to discussing what more Britain can do to help with this crisis."
Belgium's finance minister Johan Van Overtveldt warned that any demand for a British opt-out from closer union would be difficult.
He told BBC Radio 4's Today programme: "For us, Britain as part of the European Union is a very important element and we have to take into account where the British Government wants to go with this project.
"But, of course, Europe is something where you work all together and try to achieve something all together."
Asked about a formal British opt-out on closer union, he replied: "That kind of statement and that kind of policy line would of course make negotiations, to say the least, not easy."
But Government officials indicated that they do not expect any attempt to block the planned technical talks.
"There's been a sense of countries wanting to engage in this," said one official.
Downing Street said Mr Cameron and Mrs Merkel agreed in talks on Wednesday that "the next step should be to kick off technical discussions on the substance".
The European Commission has appointed a senior British official as director-general of a new Brussels taskforce to handle issues relating to the UK renegotiation and referendum, which will begin its work on September 1.
Lawyer Jonathan Faull will have responsibility for leading the Commission's response to the UK demands and will report directly to its president Jean-Claude Juncker.
Mr Cameron was also using the summit to push for progress on the creation of a digital single market in Europe, putting his name to a letter also signed by the leaders of Ireland, the Netherlands, Sweden, Poland, Finland, the Czech Republic and Estonia backing proposals drawn up by the Commission last month to bring down barriers to online trade and shopping throughout the EU.