Asked how the nerve agent works, he added: ‘It’s for paralysing people, 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.’

The Novichok family of nerve agents were secretly developed over two decades at a research facility 50 miles outside the Russian capital.