The hunt is on for the deadly Novichok-tainted item that left a couple fighting for their lives in Wiltshire.
Police said they could not rule out more people falling ill as a result of contact with the nerve agent apparently left over from the attempted murder of Sergei and Yulia Skripal.
Dawn Sturgess, 44, and Charlie Rowley, 45, were taken ill on Saturday in Amesbury, around eight miles from where former Russian spy and his daughter were poisoned with the same substance in Salisbury in March.
The second nerve agent emergency in four months has prompted a diplomatic row, with Home Secretary Sajid Javid accusing the Russian state of using Britain as a "dumping ground for poison".
The Russian Embassy hit back, accusing the Government of trying to "muddy the waters" and "frighten its own citizens".
One theory understood to be under investigation is that the pair poisoned in the latest incident may have inadvertently found a container - such as a phial or syringe - used to transport the nerve agent for the initial attack on the Skripals and discarded in a public place.
The Metropolitan Police said the Defence Science and Technology Laboratory at Porton Down confirmed on Wednesday that the victims had been exposed to Novichok.
"Following further tests of samples from the patients, we now know that they were exposed to the nerve agent after handling a contaminated item," the Met said.
"Detectives are working as quickly and as diligently as possible to identify the source of the contamination.
"Meticulous and systematic searches are under way at a number of sites. The safety of the public and our officers remains paramount and the searches will take longer because of the precautions that we must take to ensure there is no outstanding risk.
"Officers are wearing protective equipment as they carry out their activity and protective barriers may also be installed at some of these sites.
"Cordons remain in place at a number of sites in the Amesbury and Salisbury areas that we believe they visited before they fell ill. This is a precautionary measure while we continue to investigate how they came into contact with the substance."
The Chief Constable of Wiltshire Police defended his force's initial response to the latest Novichok poisoning but could not rule out that further people could fall ill.
Kier Pritchard also said he "fully supports" the decisions taken by officers who thought initially the couple had fallen ill after taking illegal drugs.
During a meeting on Thursday, Mr Pritchard said: "I hope we are not back in this position in four months' time."
Novichok remains highly toxic for a considerable period of time, so even the tiniest trace remaining in a container picked up by the victims could account for their severe illness.
In a blistering attack in the Commons, Mr Javid demanded that the Kremlin provide an explanation for the two episodes.
"The eyes of the world are currently on Russia, not least because of the World Cup," he said.
"It is now time that the Russian state comes forward and explains exactly what has gone on."
Making clear that the UK will "stand up to the actions that threaten our security", he added: "It is completely unacceptable for our people to be either deliberate or accidental targets, or for our streets, our parks, our towns, to be dumping grounds for poison."
A Russian Embassy spokesman said: "It is regrettable that UK officials try to link a second poisoning with Russia without having produced any credible results of the investigation of the first one.
"Instead of genuine co-operation, London is doing everything possible to muddy the waters, to confuse and frighten its own citizens. One can only guess what the reasons behind such a line of behaviour are."