Speaking after Theresa May warned she will walk away from any proposed Brexit deal that she cannot “defend” to the British people, Mr Juncker said negotiations were continuing.
He told reporters: “We will take the exam and look at the results so far but I don’t think there will be a miracle.
“We have some details but we don’t have all the details we need. But work is going on.
“It’s not my working assumption that we will have no deal.”
Mrs May has made clear the patience of UK voters with soaring EU demands for a multi-billion divorce payment is running out.
And she urged fellow leaders to make a “joint effort and endeavour” to hammer out a departure deal for Britain that will deliver “prosperity for all our peoples”.
Mrs May’s stark message follows growing exasperation among Brexit-backing Tory MPs and anti-Brussels campaigners at the EU’s spiralling cash demands.
One senior Eurocrat rejected Britain’s offer of a £18billion exit payment as “peanuts” earlier this week while a close ally of German Chancellor Angela Merkel yesterday suggested the figure could be as high as £80billion.
At their summit EU leaders are expected to refuse permission for the stuttering Brexit talks to move on to the crucial issue of trade, but may allow preliminary “scoping discussions” on a future trade deal to begin.
Several leaders at the summit urged the Prime Minister to provide more “clarity” about how much Britain will pay to leave the EU.
At a working dinner for the 28 EU leaders, Mrs May insisted that the UK had taken a “creative and pragmatic approach to securing a new deep and special partnership” with Brussels.
She said her speech in Florence last month setting out Britain’s offer had “taken us forward” in the effort to break the deadlock.
She had “recognised the difficulty the process was in,” she said.
“I took stock, listened to the people in the UK and my friends and partners in Europe were saying and I made a step forward.”