A "very small amount" of the Novichok nerve agent was used in the attack, with the substance delivered in a "liquid form", the Department for Environment, Food and Rural Affairs has said.
The details emerged at a press briefing in Salisbury, where reporters were told the highest concentration was found at Mr Skripal's house.
Clean-up work is set to begin at nine sites across the city, as a tenth, an area of the London Road cemetery - which was found not to be contaminated - will reopen to the public on Tuesday.
"We either know there is contamination there (in the nine sites) or we think there is a probability we could find contamination there," Defra said.
Salisbury residents have been warned to expect a step-up in activity as investigators wearing protective clothes remove items and chemically clean the areas.