Britain will respond appropriately if evidence shows Moscow sponsored a nerve agent attack on a Russian ex-spy and his daughter in southern England, Prime Minister Theresa May says in her strongest warning to date.

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 centre in the quiet cathedral city of Salisbury.

May said that 21 people have received medical treatment as a result of the attack.

British media and some politicians have speculated that the Russian state could be responsible - suggestions dismissed by Moscow as knee-jerk, anti-Russian propaganda.

Former spy Sergei Skripal shown on February 27 at a store in Salisbury, England.
Former spy Sergei Skripal shown on February 27 at a store in Salisbury, 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," May told ITV News, when asked whether Britain could expel the Russian ambassador over the attack.

"But let's give the police the time and space to actually conduct their investigation.

"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."

Scientific tests by government experts have identified the specific substance used, which will help identify the source, but authorities have refused to disclose the details.

Both victims remain unconscious, in a critical but stable condition, while a British police officer who was also harmed by the substance is now able to talk to people although he remains in a serious condition, interior minister Amber Rudd said.

"The use of a nerve agent on UK soil is a brazen and reckless act. This was attempted murder in the most cruel and public way," Rudd told parliament in a statement.

"But if we are to be rigorous in this investigation, we must avoid speculation and allow the police to carry on their investigation."