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 Salisbury on 4 March.
A massive clean-up operation is beginning to decontaminate nine sites in the city – it will take several months and cost millions of pounds.
The UK government says Russia was behind the poisoning, but Moscow denies any involvement.
Yulia, 33, left hospital earlier this month. Her 66-year-old father is said to be recovering more slowly.
The Department for Environment, Food and Rural Affairs (Defra) has released further details of the substance used and the efforts being made to decontaminate areas of Salisbury where it has been found.
A “very small amount” of the Novichok nerve agent was used in the attack, with the substance delivered in a “liquid form”, Defra said.
It will be cleaned away with caustic agents and hoardings will go up in public places in the next few days for the clean-up to begin.
The nine sites affected include ambulance stations, Sergei Skripal’s house, and various city centre locations, including Zizzi restaurant and the Mill pub, where the Skripals spent time on the day they fell ill.
Nearly 200 military personnel will help with the work, which is expected to take a number of months.
The Skripals’ poisoning has triggered a diplomatic crisis between Russia and the West, with more than 20 countries expelling Russian envoys in solidarity with the UK.