British Prime Minister Theresa May is facing a chiding from European Union leaders on Thursday over the U.K.'s slow progress in planning for Brexit.

May is due to give the 27 other a brief update on U.K. departure plans over dinner at a summit in Brussels. But Britain's looming exit is a side dish at a meeting more concerned with stemming a political crisis over migration that is shaking European unity and undermining German Chancellor Angela Merkel's government.

Organizers say May will "say a few words on Brexit from a U.K. perspective." The other 27 leaders are due to assess progress in the divorce negotiations on Friday, without May.

EU officials have warned that the timetable the two sides have set themselves — to reach a divorce agreement by October so that EU national parliaments can ratify it before Britain officially leaves the bloc in March — is slipping out of reach.

Irish Prime Minister Leo Varadkar said he expected EU leaders to tell May that negotiations "need to intensify."

"The lack of progress in the negotiations on the withdrawal agreement has been very disappointing," he said. "We still need to see detailed proposals from the U.K. on how it intends to deliver on the clear commitments it made (at previous summits) in December and March."

Divorce talks have stalled amid divisions within May's government about how close an economic relationship to seek with the EU after Brexit.

On one side are pro-EU parliamentarians who want to retain close economic ties with Britain's biggest trading partner. On the other are pro-Brexit lawmakers who want a clean break so Britain can strike new trade deals around the world.

The British leader will gather her fractious Cabinet next week at Chequers, the prime minister's country retreat, to try to draw up a united plan for future trade and security ties with the EU

So far May has fudged the issue, saying the U.K. will quit the bloc's single market and tariff-free customs union but seek trade that is "as free and frictionless as possible."

Britain has also promised to maintain an invisible border between Northern Ireland and the Republic of Ireland — the U.K.'s only land frontier with an EU member.

EU officials say they await detailed proposals from Britain for how all that can be achieved. They have warned, repeatedly, that Britain can't cherry-pick benefits of EU membership, such as access to its single market of 500 million consumers, without accepting the responsibilities, including allowing free movement of EU citizens to the U.K.