Salisbury is to be declared decontaminated of Novichok after an almost year-long military clean-up following the Sergei Skripal poisoning.
The former Russian spy’s house and 11 other potentially infected sites were expected to be ruled safe on Friday.
Military teams have spent 13,000 hours on the clean-up after Mr Skripal and his daughter Yulia were targeted with the nerve agent in March last year and left seriously ill.
During the 355-day operation, they took 5,000 test samples from across Salisbury and nearby Amesbury, where Dawn Sturgess, 44, was fatally poisoned in July.
The Skripals’ house, in Christie Miller Road, is expected to be handed from the Department for Environment, Food and Rural Affairs (Defra) back to Wiltshire Council on Friday.
Army Lieutenant General Ty Urch, who oversaw military involvement in the clean-up, said the imminent Defra announcement “will declare all of those 12 sites safe”.
Along with the house, they include the park bench where the Skripals were found collapsed, the Zizzi restaurant where they had dined beforehand, and the home of Detective Sergeant Nick Bailey, who was exposed to the agent.
Lt Gen Urch, commander home command and standing joint commander, said it had been “the longest running” operation of its kind on British soil.
He said: “Novichok is probably one of the most dangerous and most challenging chemicals in existence today and you don’t need very much of it and it’s highly spreadable.”
An estimated 600 to 800 specially trained military personnel – including the chemical, biological, radiological and nuclear regiment – were involved in the clean-up, named Operation Morlop.
Around 190 worked at any one time and there were 250 “deliberate operations” to cross into and decontaminate danger areas.
A senior military source said the Skripal house was the “most complex” but each decontamination effort was “bespoke” to the site.
Other sites were Salisbury and Amesbury ambulance stations, Bourne Hill police station, Ashley Wood vehicle recovery yard and The Mill pub.
Ms Sturgess fell ill and died after coming into contact with a perfume bottle believed to have been used in the attack on the Skripals and then discarded.
Her partner Charlie Rowley, 45, was also exposed to the nerve agent but was treated and discharged.
Mr Rowley’s home, a Boots pharmacy branch and Amesbury Baptist Church were also among the 12 sites.
In January, the roof of the Skripals’ house and garage were dismantled then removed and disposed of.
The roof was removed because the extent of the search “potentially spread the agent into every nook and cranny”, the source said.
Lt Gen Urch said ambulances used in the initial response also had to be decontaminated during the “slow, deliberate and detailed operation”.
Special dressing zones were set up near the Novichok “hotspots” where teams changed into special respirator suits which included equipment not normally used by the military, the general said.
“I think our military personnel have demonstrated genuine courage,” he said.
“This is something which young girls and boys crossing the hot zone have never done before in their lives and it’s been an amazing demonstration of physical and mental courage.”
He said the attack, believed to have been orchestrated by Russia’s secret service, was a “despicable act”, repeating a comment made by the Prime Minister in the aftermath.
Teams involved in the clean-up will be “recognised in due course for their courage”, Lt Gen Urch added.
