Hundreds of people have been told to wash their possessions as a precaution after traces of the nerve agent used to attack a Russian ex-spy and his daughter were found in a restaurant where the two were poisoned, the chief medical officer for England disclosed Sunday.

Sally Davies said that up to 500 people who have visited pubs and restaurants in the city of Salisbury should take action after former Russian spy Sergei Skripal and his daughter Yulia were sickened last week, Sky News reported.

"We have now learned there has been some trace contamination by the nerve agent in both the Mill Pub and Zizzi restaurant in Salisbury," Davies said.

"I am confident this has not harmed the health of anyone who was in the Mill pub or Zizzi's."

Davies said people who visited the restaurant after 1:30 p.m. on Sunday until evening closing on Monday should "clean the clothes they wore and possessions they handled while there."

The restaurant and pub were among five locations identified as possible sources of where the nerve agent that sickened Skripal, according to Sky News.

The sites include a bench in the Maltings shopping center where the pair were found unconscious, Skripal's home, the cemetery where his wife and son are buried, and the Bishop's Mill pub and Zizzi restaurant where the pair visited in the hours before they fell ill, law enforcement sources told Sky News.

U.K. officials haven't said what type of nerve agent was used and say the risk to the public remains low.

"However, some people are concerned long-term exposure to these substances may over weeks, and particularly months, give rise to health problems," Davies said on Sunday.

The health agency added that any clothing should be washed in "an ordinary washing machine using your regular detergent at the temperature recommended for the clothing."

Investigators in protective clothing prepare to move an ambulance at the South Western Ambulance Service station in Harnham, near Salisbury, England, as police and members of the armed forces probe the suspected nerve agent attack on Russian spy double agent Sergei Skripal, Saturday March 10, 2018. (Andrew Matthews/PA via AP)

It also said to "wipe personal items such as phones, handbags and other electronic items with cleansing or baby wipes and dispose of the wipes in the bin."

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A large-scale police investigation continues in Salisbury as forensics experts wearing protective gear search for clues. Home Secretary Amber Rudd said Saturday evening it is still "too early" to determine who is to blame for the attack.

Military personnel in gas masks prepare to remove a second ambulance from the South Western Ambulance Service station in Harnham, near Salisbury, England, Saturday March 10, 2018. (Andrew Matthews/PA via AP)

Rudd said more than 250 counterterrorism officers are on the scene evaluating more than 240 pieces of evidence and interviewing about 200 witnesses.

Police are looking for clues into what sickened Skripal, 66, a Russian ex-military intelligence specialist who in 2006 was convicted in Russia of spying for Britain, and his 33-year-old daughter, Yulia. The two remain in critical but stable condition at a hospital, officials said at a press conference on Sunday.

Senior government officials have vowed to respond robustly if the Russian government is found to be responsible.

Skripal was imprisoned inside Russia until he was freed in a 2010 spy swap and settled in England. He had stayed out of the public eye since then.

The father and daughter were found unconscious March 4 on a bench in Salisbury. Skripal lived in the town, located 90 miles southwest of London.

SPY'S POISONING IS LATEST CASE TO STIR SUSPICION OF RUSSIA

The officer who was one of the first on the scene after the Skripals were found and was sickened by the nerve agent, Wiltshire Police Sgt. Nick Bailey, thanked the public on Saturday for their their support during his recovery.

A statement released by Wiltshire Police said: "Nick would like us to say on his behalf that he and his family are hugely grateful for all the messages of support from the public, and colleagues from the police family."

Wiltshire Police Sgt. Nick Bailey, who was sickened by the nerve agent. (Wiltshire Police/Pete Brown/MoD via AP)

"He also wishes to say that he was part of a group of officers and other emergency service colleagues who dealt with the initial incident," the statement continued. "He wants to say that he does not consider himself a 'hero,' he states he was merely doing his job -- a job he loves and is immensely proud of -- just like all of his other dedicated colleagues do, day in-day out, in order to protect the public and keep people safe."

The Associated Press contributed to this report.