Ex-Russian spy in critical condition after exposure to 'unknown substance' A former spy and his daughter are in critical condition at a UK hospital.

A man believed to be a former Russian agent convicted of spying for Britain is fighting for his life in a hospital in Salisbury, England, along with his daughter, after the two came into contact with an "unknown substance" on Sunday.

The incident made international headlines after the man was named by the BBC as Sergei Skripal, one of four Russian spies involved in a prisoner exchange with the U.S. in 2010.

Local police were alerted on Sunday afternoon after Skripal, 66, and his daughter Yulia Skripal, 33, were spotted unconscious on a park bench. Sergei Skripal was displaying what witnesses described as "strange hand movements."

Counter terrorism police are leading the investigation, which is focusing on identifying the unknown substance that caused the two to become critically ill, and assessing whether there is any "criminal activity" that may have taken place.

Wiltshire police have confirmed that a number of emergency services personnel received medical attention after responding to the incident, with one person receiving ongoing treatment.

The pair "did not have any visible injuries and were taken to Salisbury District Hospital," authorities said. "They are currently being treated for suspected exposure to an unknown substance. Both are currently in a critical condition in intensive care."

The combination of a former Russian intelligence officer and contact with a potentially poisonous substance has led to comparisons with the 2006 poisoning of Russian spy Alexander Litvinenko, who died after drinking tea laced with radioactive polonium at a London hotel.

A long-running British inquiry found the Russian state to be culpable and suggested that Russian President Vladimir Putin may have ordered the assassination.

Speaking in the House of Commons on Tuesday, Boris Johnson warned that if any foreign state had taken actions on British soil, the British government would respond "robustly."

"It is too early to speculate as to the precise nature of the crime or attempted crime that has taken place in Salisbury," Johnson said. "If those suspicions prove to be well-founded, then this Government will take whatever measures we deem necessary to protect the lives of the people in this country, our values and our freedoms."

He added that the government would look at sanctions as possible punitive measures.

On Tuesday morning, Kremlin spokesman Dimitri Peskov told journalists that Moscow was willing to help in the investigation, but that Russia had no information on what could have been behind the incident.

"We see this tragic situation, but we don't have information on what could have led to this, what he was engaged in," he said.

An Italian restaurant, Zizzi's, in the center of Salisbury and The Bishop’s Mill pub in The Maltings shopping center remain closed Monday in connection to the incident.

Police officers were seen speaking to staff inside Zizzi’s on Monday night after it was closed "as a precaution," according to police on the scene who spoke with local journalists.

Skripal was sentenced in Russia to 13 years for treason in 2006 for allegedly spying for British intelligence services and passing secrets to MI6 since the late 1990s. He allegedly received more than $100,000 for sharing the identities of Russian agents in Europe.

In 2010, Skripal was exchanged with four other Russians for 10 U.S. citizens working as Russian agents under deep cover in the U.S. He was officially pardoned the following year in a decree signed by then-Russian President Dmitry Medvedev.