The identification of Novichok as the weapon has become the linchpin of the British case for Russia’s culpability in the poisoning. Though one of the scientists who developed the nerve agent, Vil Mirzayanov, published its chemical structure and now lives in the United States, it is extremely dangerous to manufacture. He said in an interview that the only laboratories known to have fine-tuned the process were in Russia and parts of the former Soviet Union.

Russian officials have strenuously argued against this point in recent days, suggesting at different times that the Skripals had not been attacked and that Britain was behind the attack.

In Moscow on Sunday, in his first extensive remarks about the poisoning, President Vladimir V. Putin called it “total rubbish, drivel and nonsense” to think that someone in Russia would carry out such “antics” so close to Russia’s presidential election, which was Sunday, and the World Cup soccer tournament, which begins in June in Russia.

Mr. Putin said he first learned about the poisoning, which he called a “tragedy,” from the news.

His initial reaction, he said, was that people would have died on the spot if it had really been a military-grade poison. He also denied that Russia had any such substance.

“We destroyed all of our chemical weapons under the control of international observers,” Mr. Putin said, speaking at a late-night news conference to celebrate his election victory. “Moreover, we were the first to do this, unlike some of our partners who promised to do this but have yet, unfortunately, to keep their pledge.”