Russian agent convicted of spying for Britain was fighting for his life last night amid suspicions he was poisoned in a shopping centre in Wiltshire.

Sergei Skripal, 66, was in intensive care after being exposed to a mysterious substance as he sat on a bench in the centre of Salisbury. A 33-year-old woman who was with him, is also in critical condition. Both had collapsed and were unconscious when they were discovered.

Reports suggest Col Skripal had recently gone to police claiming he was fearing for his life.

The incident comes a little over a decade after the murder of Alexander Litvinenko, the former Russian agent who was poisoned by radioactive polonium in a London hotel.

Colonel Skripal, 66, a former Russian intelligence agent with the FSB, was jailed in Moscow for spying for Britain but had arrived in the UK in 2010 as part of a prisoner exchange. Anna Chapman, a Russia-born secret agent who had acquired British citizenship and who was detained by US authorities, was sent back to Russia along with nine other agents.

Litvinenko’s widow Marina Litvinenko told the Telegraph last night: “It looks similar to what happened to my husband but we need more information. We need to know the substance. Was it radioactive?”

Colonel Skripal and his female companion was discovered on Sunday afternoon. An eyewitness told how she saw the pair seemingly “frozen” in place.

Georgia Pridham, 25, had been for a hen do lunch and was walking back to her friend’s car when she saw the couple slumped on a bench.

She told the Telegraph: “He was quite smartly dressed, which caught my eye. He had his palms up to the sky as if he was shrugging and was staring at the building in front of him.

“He had a woman sat next to him on the bench who was slumped on his shoulder. She looked grey and had her hood up.

“I thought ‘that is quite odd, they must be on something’. I remember looking back at him thinking he would catch my eye but he was staring dead straight. He was conscious but it was like he was frozen and he was slightly rocking back and forward.

“She was just slumped onto his shoulder. She had a Parka jacket on with her hood up. I thought maybe she was asleep or passed out. He had a jacket and some smart trousers on. He didn’t look like the type to get high.”

The couple were unconscious when they were rushed to Salisbury District Hospital. Authorities later declared a major incident and its Accident & Emergency unit had to be closed.

On Monday night police said Zizzi restaurant, which was close to where Col Skripal and the woman were found, was closed in connection with the incident following consultation with Public Health England, suggesting one or both of them may have dined there beforehand.

Police wearing protective suits on Monday night were examining the area around the bench where the couple had collapsed. One well-placed source told The Telegraph a number of police officers who had initially attended the scene had also been treated for possible contamination although this newspaper was unable to verify that.

One report suggested a ‘specialist chemical response unit’ had removed an ‘unknown substance’ which had been wrapped in several protective layers.