BRUSSELS (AP) — If European Union chief Donald Tusk is still "a dreamer" about Britain having second thoughts on leaving the EU, some other EU leaders at a summit Thursday tried to dash his hopes.

Discord over whether the exit process could still be reversed surfaced at the summit as British Prime Minister Theresa May was preparing to brief the 27 EU other leaders on the early going of the Brexit negotiations that started this week.

Tusk said that when British friends asked him if he could imagine a way for Britain to remain part of the bloc, he told them "that in fact the EU was built on dreams that seemed impossible to achieve."

"So who knows? You may say I am a dreamer but I'm not the only one," Tusk continued, quoting the often-invoked lyric from the late John Lennon's "Imagine."

Belgian Prime Minister Charles Michel immediately counted himself out. He said the will of the British people who voted in a June 2016 referendum to leave the bloc had to be respected.

"I am not a dreamer. And I am not the only one," Michel said.

The Belgian leader insisted the Brexit negotiations should proceed apace without fanciful distractions.

"What we also need is certainty, for our companies in Belgium, in Europe," he said. "If we back this image that Brexit perhaps would not happen, it brings an uncertainty."

German Chancellor Angela Merkel also was focused on imagining an EU without Britain.

"For me, shaping the future of the 27 (remaining) member states has priority over the question of the negotiations with Britain on its exit," Merkel said.