About 50 to 100 grams of liquid nerve agent was used in the March 4 attack on the former Russian spy Sergei Skripal and his daughter, Yulia, according to the director-general of the Organization for the Prohibition of Chemical Weapons.

That quantity — a range from slightly less than a quarter-cup to a half-cup of liquid — is significantly larger than the amount that would be created in a laboratory for research purposes, meaning that it was almost certainly created for use as a weapon, the director-general, Ahmet Uzumcu, said in an interview. He added that he did not know the precise amount.

He said he had taken steps to add the nerve agent, one of a series of chemicals created under the code name novichok, to the list of chemical weapons monitored by the OPCW, a global body created to oversee the elimination of stockpiles after the end of the Cold War.

After that, countries that are signatories to the Chemical Weapons Convention — like Russia, the United States and the United Kingdom — will be required to declare production or stockpiling of novichok beyond the 5 to 10 grams needed for research purposes, or to develop an antidote, he said.

It would be the first chemical added to the list since 1993, when the treaty was signed.

Skripal, a former Russian double agent, and his daughter collapsed on a bench in Salisbury, a city in southern England, several hours after they were exposed to novichok, a nerve agent that Soviet scientists developed for battlefield use against Western troops.

Investigators have said that the substance was applied to the door of Skripal's home, and that it likely seeped through their skin over the course of several hours, rendering them unconscious after they left a restaurant in central Salisbury.

Russian officials, who deny any involvement in the attack, have suggested that Western laboratories may have synthesized the poison used. Uzumcu said that if Western laboratories had produced novichok for research purposes, or to develop an antidote, they would have created a smaller quantity than what was used in the March 4 attack.