UK officials have said that a "very small amount" of the nerve agent Novichok was used to poison Sergei Skripal and his daughter Yulia in Salisbury last month.
The Department of the Environment said the substance was delivered in a "liquid form" in the attack on 4 March.
The details emerged at a press briefing in Salisbury, where reporters were told the highest concentration was found at Mr Skripal's house.
Ten sites across Salisbury have been cordoned off since the attack.
Clean-up work is set to begin at nine of them.
"We either know there is contamination there (in the nine sites) or we think there is a probability we could find contamination there," the department said.
The tenth site – the cemetery in which Mr Skripal’s wife and son are buried – was found not to be contaminated and will reopen to the public today.
Salisbury residents have been warned to expect a step-up in activity as investigators wearing protective clothes remove items and chemically clean the areas concerned.