- Video report by ITV News Correspondent Rupert Evelyn
The nerve agent used to poison Sergei Skripal and his daughter Yulia in Salisbury was delivered “in a liquid form”, the Department for Environment has said.
The novichok chemical weapon will still be present at contaminated sites, according to a Defra official, as a clean-up operation gets underway.
At a briefing in Salisbury reporters were told Sergei Skripal's house was the most contaminated site.
Only a very small amount of the extremely toxic nerve agent was used, and someone would not necessarily notice if they came into contact with it.
Direct contact would have been needed for the victims to be poisoned, the official added.
The class of nerve agent does not produce significant vapour or gas.
A clean up operation, costing million of pounds, is due to begin at nine sites where contamination is suspected.
"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.
The work is expected to take a number of months, and toxic waste will be incinerated.
Defra added it is very confident the clean-up will make the sites safe for future use, as residual traces of the agent will be eliminated.
Yulia Skripal was discharged from hospital earlier this month, but she said she is still suffering with the effects of the nerve agent used against her and her father, former double agent Sergei, who remains in hospital.