European Council President Donald Tusk on Thursday decried the recent nasty turn in rhetoric around Brexit and urged “discretion, moderation, mutual respect.”

“These negotiations are difficult enough as they are,” Tusk said at a news conference. “If we start arguing before they begin, they will become impossible.”

Tusk’s comments came a day after U.K. Prime Minister Theresa May accused Brussels of trying to interfere in the British election.

“The stakes are too high to let our emotions get out of hand,” Tusk said at the news conference following a meeting with Norwegian Prime Minister Erna Solberg.

“Because at stake are the daily lives and interests of millions of people on both sides of the Channel,” he continued. “We must keep in mind that in order to succeed, we need today discretion, moderation, mutual respect and a maximum of good will.”