British Prime Minister Theresa May has threatened to respond “properly” to Russia if evidence proves Moscow’s role in an alleged nerve agent attack on an ex-Russian spy and his daughter in England.

“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,” a diplomatic May said in a Thursday televised interview when asked whether the UK would expel Russia’s ambassador to the country.

“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,” she further told ITV News.

The 66-year-old former double agent Sergei Skripal, and his 33-year-old daughter, Yulia, have been hospitalized since they were found unconscious on a bench outside a shopping center in the city of Salisbury on Sunday.

British police on Wednesday attributed the critical illness of the two to a nerve agent.

May said police needed more time.

“Let’s give the police the time and space to actually conduct their investigation,” she said, amid widespread accusations by British media and politicians blaming Russia for the alleged attack.

Skripal betrayed dozens of Russian agents to British intelligence before his arrest in 2004, British media reported. He was sentenced to 13 years in prison in 2006, but was granted political asylum in Britain in 2010 after being exchanged with Russian spies.

Both Skripal and his daughter remain unconscious in a critical but stable condition, according to a Reuters report. Police Detective Sergeant Nick Bailey has also fallen ill because of contact with the substance while investigating the incident.

“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 Wiltshire Police Chief Kier Pritchard, after visiting Bailey in a hospital intensive care unit.

Pritchard added that a total of 21 people had been treated in hospital following the incident.

Government experts have reportedly identified the nerve agent — which could facilitate the identification of its source — but have not made it public yet.

Interior Minister Amber Rudd has said, “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.”

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

But Rudd has also cautioned against speculation.