A sign stands outside Salisbury Memorial Hospital where former Russian double agent Sergei Skripal and his daughter Yulia, were taken after being found critically ill in Salisbury, England. Picture: Matt Dunham/AP

Salisbury, England - A Russian former double agent and his daughter are still in a very serious condition in hospital five days after they were struck down by a nerve agent in a quiet English city, Britain's interior minister said on Friday.

Sergei Skripal, 66, who passed Russian secrets to Britain, and his daughter Yulia, 33, have been in intensive care since they were found slumped unconscious on a bench on Sunday afternoon in the cathedral city of Salisbury.

As counter-terrorism police continued to investigate the source of the nerve agent, Home Secretary Amber Rudd visited the city on Friday, including the area around the bench -- now covered by a police forensics tent -- where Skripal was found.

"It is still very serious for the two people who were the subject of this outrageous attack," Rudd said.

Police said Skripal and his daughter were deliberately targeted with the rare toxin. They said experts had identified the substance, which will help determine the source, but have not yet disclosed to the public what it was.

The attack has been likened in Britain to the assassination of ex-KGB agent Alexander Litvinenko, a critic of Russian President Vladimir Putin, who died in London in 2006 after drinking green tea laced with radioactive polonium-210.

Britain has said it will respond appropriately if evidence shows Russia was behind the poisoning. The Kremlin has denied any involvement in the incident and says anti-Russian hysteria is being whipped up by the British media.

"In terms of further options, that will have to wait until we're absolutely clear what the consequences could be, and what the actual source of this nerve agent has been," Rudd said.

Responding to previous comments by the interior minister, Russia's embassy in London tweeted on Thursday: "Totally agree with Secretary @AmberRuddHR: first evidence then conclusions on Mr Skripal's case. Responsible political approach."

Sergei Skripal speaks to his lawyer from behind bars seen on a screen of a monitor outside a courtroom in Moscow in 2006. File picture: Misha Japaridze/AP


A total of 21 people were taken to hospital following the incident but only one other person, Detective Sergeant Nick Bailey, who was the first police officer on the scene, is still being treated. He remains in a serious condition although he is now speaking, Rudd said.

She declined to give any details about the police investigation, saying detectives needed space to determine how and where Skripal and his daughter were poisoned.

Police have cordoned off Skripal's modest home in Salisbury, about 80 miles (128 km) from London, where a number of forensic tents were erected in the garden. Officers were guarding the area where he and his daughter were found, along with a pizza restaurant and a pub they had visited and the graves of Skripal's wife and son.

"We have to give the police all the space they need in order to collect all the information, to secure and to be able to be absolutely clear that there is no further risk," Rudd said.

Skripal betrayed dozens of Russian agents to British intelligence before his arrest in Moscow in 2004. He was sentenced to 13 years in prison in 2006, and in 2010 was given refuge in Britain after being exchanged for Russian spies.

A British public inquiry said Litvinenko's murder 13 years ago had probably been approved by Russian President Vladimir Putin and carried out by two Russians, Dmitry Kovtun and Andrei Lugovoy. Lugovoy is an ex-KGB bodyguard who later became a member of the Russian parliament.

Both denied responsibility and Russia has refused to extradite them.