Britain must agree to honour its outstanding financial commitments to the European Union before talks about trade can begin, the president of the European Council has said.
Leaders of the other 27 nations will meet on Saturday to agree their negotiating strategy for Brexit.
Donald Tusk said discussions about future relations can only start once "we have achieved sufficient progress" on key exit issues.
He said: "In other words, before discussing our future, we must first sort out our past."
The EU "will not discuss our future relations" with the UK until "sufficient progress" has been made on key issues, Mr Tusk said.
As well as outstanding financial obligations, a deal on the rights of EU citizens living in the UK and Britons living in the rest of the bloc must be reached.
Mr Tusk also called for action to avoid a hard border between Northern Ireland and the Republic.
The council president said this approach was "not only a matter of tactics, but - given the limited time-frame we have to conclude the talks - it is the only possible approach".
In a letter to EU leaders ahead of the meeting on Saturday, he wrote: "Only once we collectively determine in the European Council that sufficient progress has been made on all these issues, will we be in a position to hold preparatory talks on the future relationship with the UK.
"I would like us to unite around this key principle during the upcoming summit, so that it is clear that progress on people, money and Ireland must come first.
"And we have to be ready to defend this logic during the upcoming negotiations."