French President Emmanuel Macron warned European leaders against breaking ranks with the EU’s “united front” and pursuing their own interests in Phase 2 of the Brexit talks, the Telegraph reported Thursday.

As the two sides move into talks on the U.K.’s future relationship with the bloc, Macron said the EU27 faced the so-called “prisoner’s dilemma” — a game theory concept in which two parties acting in their own selfish interests refuse to cooperate and so ultimately lose out. “Each of us can have our own interests,” Macron said, “That’s what the prisoner’s dilemma is all about. Everyone can have an interest in negotiating on their own and think they can negotiate better than their neighbour.”

The French president said that if individual countries did that, “it is probable that collectively we will create a situation which is unfavorable to the European Union and thus to each one of us.”

Macron backed the concept of a “common mandate” that has served the EU27 well so far whereby the European Commission negotiates on behalf of national capitals. “I think this is the right method as it avoids divisions and once again allows us to preserve the collective interest,” he said.

European Council President Donald Tusk in December said he had “no doubt that the real test of our unity will be the second phase of the Brexit talks” and called for more “courage” and “realism” from member countries.

Macron will meet with British Prime Minister Theresa May at a Franco-British summit on January 18.