The nerve agent used to poison former Russian spy Sergei Skripal and his daughter was delivered in liquid form, and it will take months to remove all traces of the toxin, British officials say.
The Environment Department says nine sites need "specialist cleaning," including a restaurant visited by the Skripals on March 4, the day they were found unconscious on a park bench in Salisbury, southern England.
The department said a "very small amount" of Novichok was used with the substance delivered in a "liquid form".
Officials say the Skripals were poisoned with a military-grade nerve agent made in Russia, which was put on a door handle at Sergei Skripal's house.
Britain says it's "highly likely" Russia is behind the poisoning of Sergei Skripal and his daughter Yulia. Source: AAP
Further details about the substance emerged at a press briefing on Tuesday.
A department spokesman said: "In this instance, direct contact is required for a person to poisoned. Only a small proportion of the material is transferred in each contact and the substance is diluted in each secondary or tertiary contact.
"The class of nerve agent does not produce significant vapour or gas and can only be moved between sites by direct transfer from a contaminated person or by moving a contaminated item."
Meanwhile, work is set to begin to decontaminate the nine locations experts either know or believe are contaminated.
"We either know there is contamination there, or we think there is a probability we could find contamination there," the department said.
A multimillion-pound operation, involving around 190 specialist military personnel, is expected to start in the coming days, with the process lasting months.
Yulia Skripal, 33, was discharged from hospital a week ago while her father, 66, is no longer in a critical condition.
Britain has blamed Russia for the attack which the authorities said was carried out using a nerve agent. Moscow denies the accusation and says Britain is trying to whip up anti-Russian hysteria.
