BRUSSELS — Divided over their other pressing problems, European Union leaders on Friday took less than a minute to unite over a warning to Britain’s prime minister, Theresa May, that she risks a disorderly and disruptive British exit from the bloc unless she breaks months of political deadlock in London and makes “realistic” proposals.

With Britain planning to end more than four decades of European integration in March, crucial questions remain unresolved. Mrs. May’s divided cabinet is constraining her power to negotiate, and the thought is dawning in some capitals that Britain might end up, accidentally, leaving the bloc without any deal at all, which most experts say would be an unmitigated disaster.

After a marathon negotiation on migration that began Thursday and went well into Friday morning, the leaders dispensed with the British withdrawal, known as Brexit, in less than 60 seconds, according to the Maltese prime minister, Joseph Muscat — though they devoted longer to venting their frustration at the glacial pace of progress.