Police have tonight said that a couple from Amesbury were exposed to the nerve agent Novichok.
Metropolitan Police Assistant Commissioner Neil Basu said the substance which has left two people critically ill in Amesbury was nerve agent Novichok.
Both patients remain in a critical condition in hospital, Mr Basu said, although nobody else has presented with signs of exposure to the nerve agent.
Mr Basu said it was "a line of inquiry" over whether the latest poisoning was linked to that of former Russian spy Sergei Skripal and his daughter Yulia in Salisbury in March.
Home Secretary Sajid Javid will chair a meeting of the Government's Cobra emergencies committee tomorrow, Downing Street said.
Officers were called to a home in Muggleton Road, Amesbury, Wiltshire, on Saturday morning when the 44-year-old woman collapsed. They were called back later that day when the man, 45, also fell ill.
The pair were named locally as mother Dawn Sturgess and Charlie Rowley.
They were discovered around eight miles from where former Russian spy Sergei Skripal and his daughter Yulia were poisoned with a nerve agent in March, but it is not thought that the individuals are linked in any way to Russia or to the Skripals.
The Metropolitan Police, which leads the national counter-terrorism network, is assisting in the latest case "given the recent events in Salisbury", but declined to comment on whether they became ill due to accidental exposure to the Novichok agent.
A meeting of the Government's Cobra emergencies committee took place at official level on Wednesday morning to discuss the events and a second meeting was taking place that evening to update senior Whitehall officials.
A Downing Street spokesman said: "This is an incident which understandably is being treated with the utmost seriousness.
"Ministers and the Prime Minister are being kept updated and there was a meeting this morning of officials to receive updates on the facts of the situation."
Mr Basu said: "This evening we have received test results from Porton Down that show the two people have been exposed to the nerve agent Novichok.
"From initial assessment it was thought that the two patients had fallen ill after using drugs from a potentially contaminated batch.
"However, on Monday, 2 July, due to concerns over the symptoms the man and woman were displaying, samples from both patients were sent to Porton Down laboratory for analysis.
"Following the detailed analysis of these samples, we can confirm that the man and woman have been exposed to the nerve agent Novichok, which has been identified as the same nerve agent that contaminated both Yulia and Sergei Skripal.
"The latest update we have from the hospital is that both patients remain in a critical condition. Both are British nationals and are local to the area. Officers are still working to identify their next of kin.
"At this stage, no-one else has presented with the same symptoms linked to this incident.
"The priority for the investigation team now, is to establish how these two people have come into contact with this nerve agent."
Mr Basu also said 100 officers were working on the case and that several streets had been cordoned off.
As a precaution he said more police would be stationed on the streets.
Mr Basu added: "I would add that the complex investigation into the attempted murders of Yulia and Sergei remains ongoing and detectives continue to sift through and assess all the available evidence and are following every possible lead to identify those responsible, for what remains a reckless and barbaric criminal act.
"However, I must say that we are not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to. The possibility that these two investigations might be linked is clearly a line of enquiry for us."