Police cordons have sprung up around Salisbury as preparations begin to rid nine hot spots of the toxic after-effects of last month’s nerve agent attack.
A small amount of the Novichok substance is thought to have been used in liquid form to target Russian ex-spy Sergei Skripal, 66, and his daughter Yulia, 33.
The multimillion-pound decontamination operation will centre on nine areas where experts think traces could still linger.
Four cordons were installed overnight – at The Maltings shopping centre, The Mill pub, Riverside House and Zizzi restaurant, Wiltshire Council said.
Work is expected to begin next week and will likely prioritise a police station, two ambulance stations and the council building which were affected, it is understood.
A council spokesman said on Friday morning: “Those barriers are there because they are not starting their work over the weekend.”
It could take months for the process to be complete.
Asked whether the chemical remains at the same “lethal” level as the day it was put on Mr Skripal’s door, Defra’s chief scientific adviser Ian Boyd said: “We have to make an assumption that in certain circumstances there will be relatively high concentrations, probably in very, very specific locations, which could be at levels that could be toxic to individuals.
“But those hot spots will still be in the locations we are talking about.
“In these locations, there may well be higher concentrations that we still have to find, but we already know there are some high concentrations within those locations.”
Residents were told the Bourne Hill building, housing Salisbury’s police station as well as Wiltshire council’s offices will close for up to eight weeks from Friday.
The decontamination work will focus on the evidence room and two lockers inside the station, which were sealed off after the March 4 attack.
Deputy Chief Constable Paul Mills, from Wiltshire Police, said it would be “business as usual” as operations move to other sites in and around the city.
Other areas earmarked for chemical cleaning include two ambulance stations, a car compound and the home of poisoned police officer Nick Bailey.
Mr Skripal’s home, which is still part of the police investigation, will be the last to be decontaminated.
A Defra spokeswoman said: “As Public Health England have stated, Salisbury is safe for residents and visitors.
“All work undertaken on each site will take appropriate measures to ensure that there is no additional risk to the public.
“There is no need to take any additional precautions. Cordons are in place to protect the public from any potential risk.”
Novichok degrades slower than other nerve agents as it is very pure, an expert has said.
Alastair Hay, professor emeritus of environmental toxicology, University of Leeds, said: The agent in question was developed because it is so toxic with the high degree of purity helping to slow degradation.”
He added: “The nerve agent will be quite firmly attached to all sorts of surfaces and is unlikely to present an airborne hazard given both its persistence, and the fact that no one else, other than DS Nick Bailey, was injured. DS Nick Bailey’s contact with the agent appears more likely to have been through skin contact.
“A variety of procedures will be tested to find a suitable decontaminant to degrade the agent. Household bleach is very effective for degrading many chemical weapons including most nerve agents and is likely to be tried with this Novichok agent.
“If ineffective, there are now many other products available, with a range of properties, that will be tested to find the most suitable. As different surfaces are affected this may take some time as Defra has indicated.”
Mr Bailey has been discharged from hospital after he was left critically ill by the nerve agent when he visited the home of the Skripals.