The pair, in their 40s, were found unconscious in a property in Muggleton Road, Amesbury, Wiltshire, on Saturday.
It is around eight miles from where former Russian spy Sergei Skripal and his daughter Yulia were poisoned with the same nerve agent in Salisbury in March.
A meeting of the government’s Cobra emergencies committee took place at official level yesterday 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.”
Wiltshire Police initially thought that the couple had taken contaminated crack cocaine or heroin, but then decided to carry out further tests and yesterday declared a major incident. The Metropolitan Police said tests carried out at the government chemical weapons research laboratory at Porton Down confirmed the substance was Novichok.
Metropolitan Police Assistant Commissioner Neil Basu said: “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 priority for the investigation team now, is to establish how these two people have come into contact with this nerve agent.”
Both patients remain in a critical condition in hospital, Mr Basu added, although nobody else has presented with signs of exposure to the nerve agent.
It is believed that one of the last places they were seen in public was a family fun day at Amesbury Baptist Centre on Saturday afternoon.
The church is one of a number of locations in Amesbury and Salisbury which has been cordoned off by police.
Church secretary Roy Collins said: “Last weekend we held a community fundraiser and we understand this may well be the last event this couple went to in public.
“We are all quite puzzled and shocked – naturally the connection with Salisbury and recent events there mean there is a heightened public interest.
“We are praying for the couple. One of our members knows them and clearly there are concerns for them and any others in the community. They are not church members or regulars.”
Mr Collins said around 200 people attended the event, including many families and children, but “nobody else has suffered any ill-effects”.
Public Health England (PHE) said it did not believe there to be a “significant health risk” to the wider public, although its advice was being reviewed.
The address where the couple were found is on a new housing development on the southern edge of the town, which lies close to Stonehenge.