BERLIN — Chancellor Angela Merkel of Germany warned Britain on Thursday not to harbor “illusions” that it can negotiate a deal on future relations with the European Union while simultaneously negotiating its departure from the bloc.

Future relations can be negotiated only after the terms of the exit have been agreed upon, Ms. Merkel told Germany’s Parliament on Thursday.

“I have the feeling that some people in Britain still have illusions on this score,” she said. “That would be a waste of time.”

Ms. Merkel was not more specific, but as a leader known for choosing her words carefully, her warning about the high costs of leaving Europe could be interpreted as a subtle message for voters in France, Britain and Germany itself.