Donald Tusk has claimed Britain must agree to honour its outstanding financial commitments to the European Union before talks about post-Brexit deal can begin.
The president of the European Council said discussions about future relations can only start once "we have achieved sufficient progress" on key exit issues.
The leaders of the other 27 nations are to meet on Saturday (April 29) to agree their negotiating strategy for Brexit.
Tusk 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, he added.
Tusk said that 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, and action must be taken to avoid a hard border between Northern Ireland and the Republic.
The 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."