BRUSSELS — British Prime Minister Theresa May tried to rally European Union leaders into a unified stance Thursday against Russia, saying the poisoning of a former spy on English soil shows that Moscow poses a long-term threat to the West.

But as Russia denied responsibility and slammed Britain’s investigation into the nerve-agent attack, some European leaders urged caution while the investigation continues.

Sergei Skripal and his daughter Yulia remain unconscious in critical but stable condition after the March 4 nerve agent attack in the English city of Salisbury, which has sparked an east-west diplomatic crisis.