David Cameron faces a "make-or-break" meeting with European Union leaders on his demands for reform of the UK's relationship with Brussels, a key player in the process has said.
European Council president Donald Tusk delivered his verdict on the high-stakes nature of the summit as he arrived for the talks.
His comments came after Brussels' top official insisted he is "confident" that a deal will be reached on the Prime Minister's demands for change.
European Commission president Jean-Claude Juncker said he was "convinced" that the UK would remain a "constructive and active" member of the 28-nation bloc.
Key elements of the Prime Minister's demands for change in Britain's relations with the EU still in dispute and will be thrashed out at the meeting of the European Council which is scheduled to stretch into Friday.
Mr Tusk said: "This is a make-or-break summit, I have no doubt."
Mr Juncker said: "I'm quite confident that we will have a deal during this European Council."
But he acknowledged there were a "certain number" of issues in the reform proposals that were yet to be resolved.
A new draft of the reform package circulated to EU leaders by Mr Tusk made clear that agreement had not been reached on highly complex financial rules designed to protect the interests of members which do not use the euro.
Mr Tusk's latest proposals show only minimal progress since an earlier draft last week. Crucially, the document adds square brackets to a section relating to protections for non-euro states as the single currency integrates further, indicating that this is an issue which has not been settled in negotiations at official level and must be thrashed out by leaders round the table in Brussels.
The new text also removes a reference to the European Council using an "implementing act" to impose an emergency brake on migrant welfare at times when member states are facing extreme pressure from the volume of arrivals. An act of this kind can be used to bypass the European Parliament and its removal has sparked speculation that MEPs may be given a role in deciding whether the brake may be applied.
Also still in question is the duration of any brake. The UK initially wanted to be able to impose it for seven years, while the European Commission favoured two years with an option for a two-year extension.
Meanwhile, a provision that could have made the benefit curbs available only to Britain and other countries that did not impose transitional restrictions on migration when the EU expanded in 2004 has been removed, suggesting they will now apply to all member states.
This will be very poorly received by the Visegrad group of countries in eastern and central Europe - Poland, Hungary, the Czech Republic and Slovakia.
And eastern European states are standing firm on their insistence that changes to child benefit rules, which would cut payments to the offspring of migrant workers living in their home country, should apply only to new claims and not to the 34,000 already receiving payments form the UK.
Mr Cameron is also facing resistance to the rewriting of EU treaties to include Britain's hoped-for exemption from the requirement to seek "ever-closer union", as well as the protections for non-eurozone states.
Britain made clear that the PM is ready to walk away without agreement if he cannot secure a satisfactory package.
"This is crunch time," said one UK official. "We need to secure the right settlement for the British people."
Mr Cameron will hold further talks with Mr Tusk before the summit begins and will also meet Croatian PM Tihomir Oreskovic and Latvian PM Maris Kucinskis, both of whom have been elected since the last Council summit in December.
The first session in Brussels will provide an opportunity for each of the 28 member states to set out their views on the UK demands.
Officials are then expected to work late into the night hammering out the shape of a possible final deal, while leaders discuss the migration crisis over dinner.
On Friday morning - in what Brussels wags are calling the "English breakfast" - the leaders will return to the table to thrash out remaining points of difference and determine whether they can craft a package acceptable to all.
Former Labour leader Lord Kinnock, who was a European commissioner, said Mr Cameron appeared to have secured as much as he could have and warned of "seismic" consequences if the UK left the EU.
Vote Leave said the reforms were "trivial" and claimed any deal would have "no more legal weight than an unsigned contract".
European Parliament president Martin Schulz, who will join leaders at the table on Friday, said MEPs will work "constructively" to pass the legislation necessary to put any deal into effect.