A scientist who revealed Russia's secret chemical weapons experimentation in the 1990s has described the "horror" faced by victims exposed to the Novichok nerve agents.

Vil Mirzayanov said the effects of the poison could amount to "torture" - and claimed that the use of the lethal toxin on former spy Sergei Skripal and his daughter Yulia was a "brazen attack".

The chemist hit the headlines in 1992 after alleging in a newspaper article that Russia had been developing a particularly lethal new nerve gas.

Mr Mirzayanov, who was later arrested but now lives in the United States, had worked in a secret laboratory which was developing the Novichok nerve agents.

He told the Daily Mail that the toxins were "for paralysing people", adding: "It causes you convulsions and you can't breathe and after that you die. If you get enough of a dose of it.

"It's real torture, it's impossible to imagine. Even in low doses the pain can go on for weeks. You cannot imagine the horror, it's so bad."

Novichok, which means newcomer in Russian, was developed by the Soviet Union over several decades as a new kind of chemical weapon that would be harder to detect, and more potent than existing nerve agents.

Mr Mirzayanov said "around 1,000 people" worked across several laboratories, testing and developing the toxins.

After his newspaper article, the scientist was accused of betraying state secrets, but a subsequent trial collapsed after investigators did not find enough evidence to back charges that he had broken the law.