A 45-year-old British man who was exposed to a high dose of the nerve agent Novichok is no longer in critical condition, the hospital treating him said on Wednesday.
"He is no longer in a critical condition," Lorna Wilkinson, director of nursing at Salisbury District Hospital in southwest England, said in a statement.
Wilkinson said Charlie Rowley's condition had improve overnight and was now "serious, but stable".
"Charlie still has some way to go to recover, but the progress we've seen so far gives us cause for optimism."
Police said in a statement that they had spoken to Rowley.
"Officers from the investigation team have spoken briefly to Charlie and will be looking to further speak to him in the coming days," the statement said.
The hospital on Tuesday said Rowley, who fell ill on June 30 at his home in Amesbury near Salisbury, had regained consciousness but was "not out of the woods yet".
His partner who was also contaminated, Dawn Sturgess, 44, had collapsed earlier in the day on June 30 at his house. She died on Sunday.
Sam Hobson, a friend of the couple, told AFP he had visited Salisbury with them the day before they fell ill.
Hobson said he went to Rowley's house on Saturday as Sturgess was being taken to hospital and stayed with him for several hours until he too began to complain of feeling ill.
"He was sweating loads, dribbling, and you couldn't speak to him," Hobson said.
"It's like he was in another world, hallucinating."
Salisbury is the city where former Russian spy Sergei Skripal and his daughter Yulia were poisoned with the same Soviet-made nerve agent four months ago.