LONDON (Reuters) - Britain will respond appropriately if evidence shows Moscow was behind a nerve agent attack on a former Russian spy and his daughter in England, Prime Minister Theresa May said on Thursday.

Former double agent Sergei Skripal, 66, and his daughter Yulia, 33, have been in hospital since they were found unconscious on Sunday on a bench outside a shopping center in the quiet cathedral city of Salisbury.

“We will do what is appropriate, we will do what is right, if it is proved to be the case that this is state-sponsored,” May told ITV News, when asked whether Britain could expel the Russian ambassador.

“But let’s give the police the time and space to actually conduct their investigation,” she added, in her first comments on the attack since police said on Wednesday a nerve agent was used.

Government experts have identified the substance, which will help identify the source, but have not made the information public.

“Of course, if action needs to be taken, then the government will do that. We’ll do that properly, at the right time, and on the basis of the best evidence,” May said.

Both victims remain unconscious, in a critical but stable condition, while police officer Detective Sergeant Nick Bailey, who was also harmed by the substance, remains in a serious condition.

“He’s well, he’s sat up. He’s not the Nick that I know but of course he’s receiving a high level of treatment,” said Kier Pritchard, head of Wiltshire Police, after visiting Bailey in a hospital intensive care unit.

“He’s very anxious, he’s very concerned. He did his very best on that night.”

A total of 21 people had been treated in hospital following the incident, Pritchard said.

“BRAZEN AND RECKLESS”

In parliament, several lawmakers defied interior minister Amber Rudd’s plea to avoid speculation about who was responsible for the “brazen and reckless act”. Some called for investigations to be re-opened into the deaths of Russian exiles in Britain in recent years.

Rudd urged people to keep a cool head and focus on the Salisbury incident.

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“This was attempted murder in the most cruel and public way. We will respond in a robust and appropriate manner once we ascertain who was responsible,” Rudd said.

“We are committed to do all we can to bring the perpetrators to justice, whoever they are and wherever they may be.”

Skripal betrayed dozens of Russian agents to British intelligence before his arrest 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.

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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.

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

Both denied responsibility and Russia has refused to extradite them.

Rudd denied that Britain’s response after Litvinenko’s murder had been too soft, effectively sending a message that such crimes could be carried out with impunity.

“We are absolutely robust about any crimes committed on these streets in the UK. There is nothing soft about the UK’s response to any sort of state activity in this country,” she told BBC radio.

In Russia, some media took an almost jocular tone.

“Do not choose England as your next country of residence, whether you are a professional traitor to your homeland, or you just hate your country in your free time,” said Kirill Kleimenov, news anchor on Russia’s state-run Channel One, as he introduced an item about the story on the evening news bulletin.

“Something’s not right there, maybe it’s the climate, but in the past few years there have been a lot of strange occurrences there with a terrible outcome. People hang themselves, fall out of windows, are poisoned, or crash in helicopters in simply industrial quantities.”