Sergei Skripal and his daughter, Yulia, will either die or be crippled by their exposure to Novichok, according to the whistleblower who alerted the world to Russia's secret chemical weapons programme.
Dr Vil Mirzayanov, a chemist who worked at the heart of the Soviet programme, said Russia was the only country able to produce and deploy such a powerful nerve agent. He said many more people may fall ill.
"It is at least 10 times more powerful than any known nerve agent. Plus practically it is incurable," he said at his New Jersey home yesterday.
"These people are gone - the man and his daughter. Even if they survive they will not recover. That is the terrible damage it does.
"I'm afraid many more people were exposed."
He said he believed the poison used in the Salisbury attack would have been manufactured in Russia as two, harmless components. They would have been taken into Britain and then combined inside a tiny, easily hidden aerosol spray that could be used to deliver a deadly dose and a "deliberate demonstration" to Moscow's enemies around the world.
Mirzayanov said even the existence of Novichok, let alone its formulae, had been a closely guarded secret, making it "unthinkable" that another country or terrorist group had been allowed access or help in its manufacture.
"Only Russia could do this," he said. "They would never give it away."
The Russian scientist, who fled his homeland in 1995 and now lives in New Jersey, revealed the existence of the Novichok family of nerve agents in 1992 but said it was still so little understood that it had never been banned by the Chemical Weapons Convention. Nor had it ever been declared by Russia. That made it perfect for assassinations, he said.