"She is incredibly scared… The police don’t even know she exists. Only a very small number of people know what went on between them and what was said. She does wonder if she should have gone to the police but decided not to, as so few people know she even knew him," a mutual friend of Skripal and the woman was quoted as saying by The Sunday Telegraph newspaper.
The newspaper added that the woman was believed to live in Salisbury and it was not known whether she might have information about the days leading to the attack or noticed any suspicious activity near Skripal's house.
Skripal and his daughter Yulia have been in hospital in a critical condition since March 4, being treated for exposure to what the UK experts say was A234 nerve agent. The UK side claimed it was a Novichok-type nerve agent, which was developed in the Soviet Union.
The Russian side also expelled UK diplomats in response and ordered the British Council to stop its activities in Russia.