The EU needs a "serious British response" to its demands to protect citizens' rights, EU President Donald Tusk said Saturday, setting terms for launching new ties with the bloc after Brexit.

"We need a serious British response," Tusk told reporters after a summit where the leaders of the remaining 27 EU countries agreed their Brexit negotiating guidelines.

"I want to assure you that as soon as Britain gives real guarantees for our citizens we will find a solution rapidly," Tusk said.