The nerve agent used to poison former spy Sergei Skripal and his daughter Yulia was delivered “in a liquid form”, the Department for Environment says.
The pair were discovered unconscious on a bench in the Wiltshire city of Salisbury on 4 March.
Yulia, 33, left hospital earlier this month. Her 66-year-old father is said to be recovering more slowly.
The UK government says Russia was behind the poisoning, but Moscow denies any involvement.