After declaring a major incident police sealed off all the areas the couple had been in the hours before they fell ill, including Queen Elizabeth Gardens, the Muggleton Road address, the Boots branch and the Amesbury Baptist Centre.

Police also sealed off Ms Sturgess's home in John Baker House, a property used by social services to accommodate people with drug and alcohol problems, which is just a two minute walk from Zizzi restaurant, where the Skripals ate before collapsing from the effects of Novichok.

Mr Skripal, a former double agent and daughter Yulia, who had been visiting him from Moscow, spent two months in hospital after being poisoned with Novichok.

They were discharged in May and moved to a secure location where they are continuing to recover.

Downing St treats incident with 'utmost seriousness'

A meeting of the Government's Cobra emergencies committee took place at an official level in the Cabinet Office on Wednesday morning to discuss events in Amesbury.

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."

A Government source said: "Senior Whitehall officials are being kept informed of what is going on on the ground as we seek to establish the facts of this case. No definitive conclusions have yet been established."

Friend who spent day in Salisbury with couple

Sam Hobson, a friend of the couple, said that the day before the pair collapsed he went into Salisbury town centre with them to visit shops and have a drink in Queen Elizabeth Gardens.

These were later sealed off by police, along with the Boots and the Amesbury Baptist Centre.

Mr Hobson said Ms Sturgess, who has known Mr Rowley for nearly a year, is not a drug user. Mr Rowley and Ms Sturgess, who are unemployed, each have a young daughter from different relationships. “I just hope they pull through this,” said Mr Hobson.

Mr Hobson told how the day before Mr Rowley and Ms Sturgess fell ill, the three of them had visited several locations in Salisbury close to the riverside bench at the Maltings shopping centre where the Skripals collapsed after succumbing to the effects of Novichok nerve agent.