UK races to find source of poison used against Russia ex-spy

British detectives on Thursday scrambled to find the source of the nerve agent used in the attempted murder of a Russian former double-agent and his daughter.

Sergei Skripal, 66, who moved to Britain in a 2010 spy swap, is in a critical condition in hospital along with his daughter Yulia after they collapsed on a bench outside a shopping centre on Sunday.

A policeman also fell ill after coming to their aid but is showing signs of recovery, according to interior minister Amber Rudd.

"The two targets are still in very serious condition, the policeman is talking and is engaging so I'm more optimistic for him, but it's too early to say," she told ITV1's Good Morning Britain.

The brazen poisoning in the southwestern English city of Salisbury is already being linked with Russia by British politicians and the media, sparking an angry response in Moscow.

- Russia suspected -

On Wednesday, British police confirmed for the first time that a nerve agent was used and that their probe was now an attempted murder investigation.

"Police are now in a position to confirm that their symptoms are a result of exposure to a nerve agent," said the Metropolitan Police.

"Scientific tests by government experts have identified the specific nerve agent used which will help identify the source."

The Times newspaper reported on Thursday that Skripal's condition was particularly severe, quoting a senior unnamed British government official.

"The feeling is that he is not going to make it out of this," the source told the newspaper. "I think it could be more positive (for Yulia)."

Police have cordoned off an Italian restaurant and a pub that the pair are believed to have visited.