Parts of Salisbury contaminated with nerve agent will be sealed off behind high-security barriers for months while they are made safe, officials have revealed.
The restaurant and pub visited by Sergei Skripal and his daughter Yulia after they were poisoned will be surrounded by semi-permanent hoarding for the work, while cordons could widen in other areas.
Around 190 specialist military personnel from the Army and RAF will support the operation, which is expected to cost millions of pounds.
At a briefing in Salisbury city hall, a Department for Environment, Food & Rural Affairs (Defra) official revealed that the nerve agent was delivered in a liquid form and spread around the city by infected people.
“In this instance, direct contact is required for a person to be poisoned – only a small proportion of the material is transferred and the substance is diluted in each secondary and tertiary contact,” he added.
“The class of nerve agent does not produce significant vapour or gas, and can only move between sites by direct transfer by a contaminated person or item.”
The official confirmed that novichok “doesn’t just disappear”, meaning that if the substance was detected in the ongoing police investigation it will still be present.
The “primary site of contamination” is Mr Skripal’s house and eight other sites are believed to have been contaminated, including Zizzi restaurant and The Mill pub which the former double agent visited with his daughter, police and ambulance stations and Detective Sergeant Nick Bailey’s home.
Public Health England (PHE) reassured members of the public that the risk to health remained low, and said sites would be chemically cleaned and retested before being reopened.
Defra’s chief scientific adviser Ian Boyd, who is chair of the decontamination science assurance group overseeing the work, said: “Our approach is based on the best scientific evidence and advice to ensure decontamination is carried out in a thorough and careful way.
“Our number one priority is making these sites safe for the public, so they can be returned to use for the people of Salisbury.
“Thanks to detailed information gathered during the police’s investigation, and our scientific understanding of how the agent works and is spread, we have been able to categorise the likely level of contamination at each site and are drawing up tailored plans.
“Meticulous work is required and we expect it will be a number of months before all sites are fully reopened.”
Defra officials said there are currently no plans to destroy any affected buildings, including Mr Skripal’s house, where police believe he was poisoned with novichok smeared on the door handle.
Investigators are still working at the home and The Mill pub, but all other sites have been released by the Metropolitan Police for decontamination work to begin.
Officials said they have a clear understanding of how the chemical works and is spread, and will be using a caustic cleaner to break it down and remove all traces before a panel approves
Testing is able to detect levels of novichok 100,000 times smaller than an acute dose, and contamination is said to be low in most areas.
Public service sites, including two ambulance stations, will prioritised, followed by closed off areas of The Maltings shopping district.
Representatives from Wiltshire Council said they wanted Salisbury to return to normal as soon as possible, and are supporting local businesses to mitigate a slump in tourist numbers.
The London Road cemetery was to become the first site to be reopened on Tuesday after testing found no decontamination present. Government agencies hope all decontamination work will be completed by the end of the year.
Skripal attack aftermath – in pictures
Alistair Cunningham, chair of the recovery coordinating group at Wiltshire Council, said residents will see more military and civic personnel wearing protective clothing moving around the city to carry out the work.
“This is being taken seriously – Defra, the Ministry of Defence and other agents in government are putting in all of the resources required to make sure the work is done thoroughly,” he told The Independent.
“It’s not a quick fix – it’s a proper clean, and that should be a reassurance.”
Ms Skripal, 33, has been released from hospital but said she was still suffering the effects of the nerve agent, while her 66-year-old father remains in a serious condition.
The government accused Russia of culpability in the attack after identifying novichok as the weapon used – analysis that was confirmed by international inspectors with the Organisation for the Prohibition of Chemical Weapons (OPCW) – and said the country’s intelligence agencies had been spying on the Skripals for at least five years.
The class of nerve agents was created by the former Soviet Union, but Vladimir Putin’s government has denied ever developing it – and also said all chemical stockpiles was destroyed.
The Kremlin has denied any involvement in the attack on Mr Skripal, who moved to England following a high-profile spy swap in 2010, but it has sparked the expulsion of Russian diplomats by Britain and its allies.
In a joint statement issued on Tuesday, the foreign ministers of G7 countries, said it was “highly likely that the Russian Federation was responsible for the attack and that there is no plausible alternative explanation”.
“We call on Russia to urgently address all questions related to the incident in Salisbury,” added the ministers, representing countries including the US, UK, Canada, France, Germany, Italy and Japan.
“Any use of chemical weapons by a state party, under any circumstances, is a clear breach of international law and a violation of the Chemical Weapons Convention. It is a threat to us all.”
In the wake of a technical alert about “malicious” Russian cyber activity issued by British and American security services, the G7 said it would protect the “rules-based international system”.
“We stand in unqualified solidarity with the United Kingdom,” the statement said. “The G7 will continue to bolster its capabilities to address hybrid threats.”
Tensions have worsened further in the wake of an alleged chemical attack by Bashar al-Assad’s forces on the rebel stronghold of Douma in Syria, sparking British, American and French air strikes on Russia’s ally.