Britain will respond "appropriately" if a foreign state is found to have been involved in the poisoning of a former Russian spy in the UK.

"This is a police investigation and it will be evidence-led and we must go where the evidence takes us," Finance Minister Philip Hammond told the BBC.

"So, we have to allow the police investigation to run its course. But if there were to be an involvement of a foreign state evidenced by this investigation, then obviously that would be very serious indeed and the government would respond appropriatey."

The facts

Former double agent Sergei Skripal and his daughter Yulia were left in a critical condition on March 4 when they were found unconscious on a bench in the southern English cathedral city of Salisbury.

British police say a nerve agent was used but have not said publicly which one.

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. In 2010, he was given refuge in the UK after being exchanged for Russian spies.

The speculation

The British media is rife with speculation that Russia could have played a part in the attack.

What has the British government said?

On Saturday, Interior Minister Amber Rudd said it was too early to say who was responsible.

Are there any concerns about contamination?

Yes, some. The BBC has reported that traces of the nerve agent had been found in Zizzi, an Italian restaurant where the Skripals had a meal before they were found unconscious. A police spokesman declined to comment.

Public Health England have advised anyone who had visited the restaurant or The Mill pub in Salisbury on the afternoon of March 4 or on the following day to wash the clothing they were wearing. Personal items like phones and handbags should be wiped clean.

"While there is no immediate health risk to anyone who may have been in either of these locations, it is possible, but unlikely, that any of the substances which has come into contact with clothing or belongings could still be present in minute amounts and therefore contaminate your skin," it said.