David Cameron is to set out his EU renegotiation aims at a meeting of European leaders in Brussels later.
The European Council summit will be the first time the objectives have been collectively discussed by EU leaders.
However, with the agenda set to be dominated by the Greek debt and migrant crises, time will be limited for leaders to consider the UK’s proposals.
The PM hopes it will pave the way for official negotiations to begin on the terms of the UK’s EU membership.
Mr Cameron wants to make changes, before putting a revised package to a referendum of the British public by the end of 2017.
He has not set out the full details of his negotiating aims but his priorities include restrictions to welfare entitlements, greater powers for national Parliaments and an opt-out for Britain from the principle of “ever closer union”.
Ahead of the summit, Mr Cameron said: “This presents an opportunity to get the negotiation under way and to kick off a process to work through the substance and to find solutions.
“It will take us another step closer to addressing the concerns that the British people have about the EU.
“And closer to changing the status quo for the better and then giving the British people a say on whether the UK should stay in or leave the EU.”
By BBC political correspondent Ben Wright
It is a big moment. David Cameron has already sketched out his plans to European leaders, hearing where they agree and where they don’t.
But reaching a deal is likely to be a long and difficult process.
This meeting is likely to give the green light to the first stage of the formal renegotiation – talks between officials in Whitehall and Brussels.
But the pressing issue for this summit is the surge in migration from outside Europe. EU leaders agree there’s a crisis. They don’t agree what to do about it.
There seems to be little support for dispersing non-EU migrants with a quota system and Britain is opting out of a programme to share the burden with other EU countries.
But David Cameron is expected to say more must be done on returning people without a legitimate asylum claim to the countries they’ve come from.
On the eve of the summit, Mr Cameron had talks with German Chancellor Angela Merkel in Berlin as part of his efforts to speak face-to-face with his European counterparts about his aims.
On Wednesday evening, during a state banquet for the Queen, who is visiting Germany, German President Joachim Gauck said the EU “needs Britain”.
“We know that we need an effective European Union based on a stable foundation of shared values,” he said.
“A constructive dialogue on the reforms Britain wants to see is therefore essential. As a good partner, Germany will support this dialogue.”
However, French minister Emmanuel Macron has told the BBC the UK should not be able to cherry-pick aspects of EU membership.
“I don’t understand how it is possible to say ‘We, the UK, have all the positive aspects of Europe but don’t want to share any of the risk with any member states’.”
At the state banquet, the Queen spoke of the need for unity in Europe and how the continent must strive to “maintain the benefits of the post-war world”.
She warned in her speech that “division in Europe is dangerous”.
Buckingham Palace officials later stressed that the Queen’s comments were not a reference to Mr Cameron’s EU reforms or upcoming referendum, but rather about the risks of wider differences dividing the continent.
EU referendum in focus
David Cameron is starting renegotiation of the terms of Britain’s EU membership ahead of a referendum. Here is some further reading on what it all means:
The European Commission has appointed a senior British official to head a new Brussels task force to handle issues relating to the UK referendum, which will begin its work on 1 September.
Jonathan Faull, who has worked in the commission for more than 30 years, will report directly to Commission President Jean-Claude Juncker.
Thursday’s summit will be dominated by discussions on the Greek crisis and the situation of migrants crossing the Mediterranean by boat for Europe.
The recent chaos at Calais, where hundreds of migrants tried to board lorries to the UK during a strike, is also likely to feature.
As a result, the UK’s renegotiation demands are not due to be considered at the working sessions of the two-day summit, according to the official agenda.
Instead, the PM will set out his aims at a working dinner on Thursday evening.