Donald Tusk says talks must prioritise interests of the remaining countries
Britain cannot expect access to the single market without accepting free movement of people, the European Commission president has said.
Jean-Claude Juncker, who is in Bratislava for the first summit of EU leaders without Britain, suggested there was no prospect of compromise between the two.
Mr Juncker said: "There is a clear interlink as we made clear at the very beginning between the access to the internal market and the basic principles of the internal market - namely the free movement of workers and we are sticking to that position.
"This is not a game between prime ministers leaving and prime ministers remaining, this is about people in Europe.
"So I cannot see any possibility of compromising on that very issue.
"We want to have very good, very close relation with the UK. At the same time, it is not possible for these negotiations to damage our interests,"
Donald Tusk, who chaired the informal EU meeting in the Slovakian capital, added that Brexit negotiations must prioritise the interests of the 27 remaining countries of the bloc and "not the leaving country".
A future trade deal with Britain was not on the agenda at the summit, which focused on the challenges the EU faces as a result of the Brexit vote - especially from migration.
Mr Tusk confirmed exit talks with the UK could not begin without a formal notification by the British government.
He revealed British Prime Minister Theresa May had led him to expect, at their meeting last week in London, that this would "quite likely" be in January or February next year.
In a sign of how EU leaders remained at odds on how to move forward as a bloc without Britain, Italian prime minister Matteo Renzi broke ranks and did not participate in a joint press conference with France and Germany which focused on how united the EU are.
He said: "I can't give joint press conference with Merkel and Hollande. I don't follow a script to make people believe we all agree."
While the message from the EU was that free movement of people in Europe would remain, the main item on the agenda was migration from outside the bloc.
Mr Tusk had told EU leaders last year's crisis was a "tipping point" in voters' distrust of the EU and that they must never again lose control of the EU's external border.
The roadmap produced by the summit had limited measures on beefing up the EU's border force particularly on Bulgaria's border with Turkey, and measures to work together on terrorism and economic growth.
But EU countries are deeply divided on the migration issue, with the central and eastern European countries deeply opposed to taking quotas of refugees as demanded by the EU, and blaming Germany for the migrant crisis.