The deadly nerve agent used to poison former spy Sergei Skripal and his daughter Yulia was delivered "in a liquid form", BBC reported quoting the Department for Environment.
Only "a very small amount" was used on the pair, who were found unconscious on a bench in Salisbury on 4 March.
The Department for Environment, Food and Rural Affairs (Defra) said the highest concentration of military grade Novichok nerve agent was found at Mr Skripal's house, but eight other sites are known, or thought, to be contaminated. They include ambulance stations and a police car compound, as well as a Zizzi restaurant and the Mill pub, where the Skripals spent time on the day they fell ill.
The nerve agent does not evaporate or disappear over time, experts said, and intense cleaning with caustic chemicals is required to get rid of it. Hoardings will go up in public places in the next few days for the clean-up to begin.
Grass and stones will be removed from public areas around the Maltings shopping centre, and all material found to be contaminated will be incinerated. Almost 200 military personnel will help with the work, which is expected to take a number of months.