BRUSSELS — When leaders of the European Union gathered last Thursday night for a working dinner inside their lavish new headquarters, the conversation turned to Russia. Over scallops and lamb, Prime Minister Theresa May of Britain shared intelligence detailing the high probability that the Russian state had carried out the poisoning of a former Russian spy on British soil.

Usually, Mrs. May is the odd woman out at European Union gatherings, given that she is trying to negotiate Britain’s departure from the bloc. But not this time.

By the next morning, European Union leaders agreed that a coordinated response was needed, according to four senior European officials. Then President Emmanuel Macron of France said everyone should go home and consider expelling Russian diplomats.

“Let’s do it on Monday at 3 p.m.,” the French president said, according to one of the senior officials, all of whom asked for anonymity given the sensitivity of the issue.