Britain blames Russia for the March 4 poisoning of former double agent Sergei Skripal and his daughter Yulia in Salisbury with a Novichok-class nerve agent, a claim that Moscow denies. Leonid Rink, a chemist who served time for supplying a military-grade poison that was used to murder a Russian banker and his secretary in 1995, has made headlines this week for acknowledging that he had helped create the nerve agent.

A Soviet-era scientist claims to have sold a deadly nerve agent in the 1990s that Britain says poisoned an ex-spy and his daughter, classified documents obtained by a Russian investigative outlet have revealed.

"Yes, I understood these people planned to use the substance against people," Rink said in a classified 2000 testimony published by the Novaya Gazeta investigative newspaper on Thursday.

Top-secret case materials into the 1995 poisoning of Russian banking magnate Ivan Kivelidi and his assistant say the investigation “has established that Rink sold [the substance] to Chechens in Moscow.”

The classified materials say that Rink sold the poisonous substance to the Chechen criminal groups a month after Kivelidi had been murdered.

Rink handed over hundreds of deadly doses of the poison that was used in the 1995 murder in an ampoule hidden inside a pen presentation box at a train station in central Moscow, according to his statement that the Reuters news agency reported on last week.

Rink said he was paid up to $1,800 for the sale, according to Novaya Gazeta.

A top-secret chemical research facility in the Russian town of Shikhany continued the development of a Novichok-type substance until 1994, the interrogation file cites an unnamed chemist who was present at Rink’s testimony as saying.

On Thursday, a London court granted permission for blood samples to be taken from the Skripals for examination by chemical weapons inspectors in the ongoing investigation.