The U.K. Environment Department said 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 the city in southwest England.
About 190 specially trained troops are assisting environmental, health and defense officials with the cleanup, it said.
"Work to clean each site will involve a process of testing, removal of items which may have been contaminated, chemical cleaning and retesting," the agency said in a statement, adding the effort will take "a number of months."
British officials say the Skripals were poisoned with a military-grade nerve agent that was smeared on a door handle at Sergei Skripal's house.
Father and daughter were hospitalized for weeks in critical condition. Yulia Skripal was released last week from Salisbury District Hospital, where her father continues to be treated.
British officials say the Russian government was behind the attack using a Soviet-developed type of nerve agent known as Novichok. The international chemical weapons watchdog, the OPCW, has confirmed that a Novichok agent "of high purity" was used, although it has not identified where it was made.
Russia denies involvement in the attack and has accused Britain of failing to share its evidence.
The poisoning has sparked a Cold War-style diplomatic crisis between Russia and the West, including the expulsion of hundreds of diplomats from the two sides.
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