Traces of the nerve agent used to attack a Russian ex-spy and his daughter have been found in Salisbury where the two were poisoned, British media says.
BBC and Sky News said on Sunday that traces have been detected.
BBC said its sources say traces were found at the Zizzi restaurant where Sergei Skripal and his daughter Yulia reportedly ate before falling critically ill last Sunday afternoon.
Sky says traces were found in several locations.
UK officials haven't publicly confirmed that any trace of the nerve agent has been found and say the risk to the public remains low.
Officials haven't said what type of nerve agent was used.
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A large-scale police investigation is underway in Salisbury as forensics experts wearing protective gear search for clues. Cordons remain in place at a host of locations across the English city, including Mr Skripal's house and the cemetery where his wife and son are buried.
British Home Secretary Amber Rudd said there were more than 250 officers from counter-terrorism units involved in the investigation, but it was still too early to say who was responsible for the attack.
"There is also substantial amounts of CCTV they have to go through. This is a painstaking, detailed investigation and the police need to be given the space and time to get on with it."
Mr Skripal and his daughter are still fighting for their lives after being exposed to a toxic substance.
Police said 21 people had been seen for medical treatment since the incident.
Meanwhile finance minister Philip Hammond said on Sunday that Britain's government would respond "appropriately" if a foreign state was found to be involved in the poisoning.
"This is a police investigation and it will be evidence-led and we must go where the evidence takes us," Mr Hammond told BBC television.
"So we have to allow the police investigation to run its course. But if there were to be an involvement of a foreign state evidenced by this investigation, then obviously that would be very serious indeed and the government would respond appropriately," he said.