UK authorities have said traces of the nerve agent used in the attack on a former Russian spy in Salisbury have been found at a pub as well as a restaurant.
Up to 500 diners and pub-goes who visited either of the premises have been urged to wash clothing and possessions.
Public Health England issued the advice as a precaution, but said the risk to the general public remains "low".
Dr Jenny Harries, PHE Deputy Medical Director, said: "The immediate risk to the general public remains low and this has not changed.
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"Rigorous scientific analysis has been on-going and we have learnt that there has been some limited contamination in both The Mill pub and Zizzi restaurant in Salisbury.
"Anyone who visited The Mill pub or Zizzi restaurant where the two affected individuals were can be reassured that this limited exposure will not have harmed their health to date.
"However, there may be a very small health risk associated with repeated contact with belongings which may have been contaminated by this substance.
"We therefore recommend that a precautionary approach is taken and advise people to clean the clothes they were wearing and any possessions they had with them."
Anyone who was in The Mill pub between 1.30pm last Sunday and 11.10pm on Monday, or the nearby Zizzi restaurant between 1.30pm on Sunday and 9pm on Monday, have been urged to follow the advice.
Russian double-agent Sergei Skripal and his daughter Yulia are still fighting for their lives after being exposed to a toxic substance in the city last Sunday, 4 March.
Detective Sergeant Nick Bailey, who was part of the initial response by authorities, is also in a serious condition.
Yesterday, police said they have identified more than 200 witnesses and 240 pieces of evidence.
Following a meeting of the British government's Cobra emergency committee, UK Home Secretary Amber Rudd said there were more than 250 officers from eight out of 11 of the country's counter-terrorism units involved in the investigation.