British Prime Minister Theresa May has accused Russia of attempting to assassinate a former spy and his daughter, saying it is “highly likely” Moscow was behind the attack.

Ms May made the damning claim in an angry address to UK Parliament on Tuesday morning (AEDT) as she revealed the nerve agent behind the attempted murder of Sergei Skripal and his daughter Yulia was a military-grade substance developed by Russia.

The Prime Minister said, given the facts uncovered in the investigation into the incident, it was “highly likely” Russia was responsible for the attack in the city of Salisbury.

The Foreign Office has asked Russia’s ambassador for an explanation.

Ms May said the UK would conclude the murder attempt was an “unlawful use of force” by Russia if there was no “credible response” from Moscow by the end of Tuesday local time. 

She told MPs the chemical used in the attack was identified as being part of a group of nerve agents known as Novichok, developed by Russian scientists.

“Either this was a direct action by the Russian state against our country, or the Russian government lost control of its potentially catastrophically damaging nerve agent and allowed it to get into the hands of others,” Ms May said.

She said Foreign Secretary Boris Johnson had asked the Russian ambassador to provide “full and complete disclosure” of the Novichok program to the Organisation for the Prohibition of Chemical Weapons.

Ms May warned the UK must be ready to take much more extensive measures, and these would be set out in Parliament on Wednesday.

Mr Skripal, 66, and his 33-year-old daughter, remain in a critical but stable condition after being found slumped on a bench in Salisbury city centre last week.

Detective Sergeant Nick Bailey, who fell ill after attending the pair, remains seriously ill in hospital but has been talking to his family.

Mrs May told Parliament the nerve agent behind the attack was identifed by the UK’s Porton Down laboratory.

She said Russia previously produced the agent and would still be capable of doing so.

The decision to point the finger at Moscow was also based on “Russia’s record of conducting state-sponsored assassinations and our assessment that Russia views some defectors as legitimate targets for assassinations”, the PM added.

Russian Foreign Ministry spokeswoman Maria Zakharova said Ms May’s statement was “a circus show in the British parliament”.

“The conclusion is obvious – it’s another information and political campaign based on provocation,” she was quoted as saying by the BBC.

Earlier, asked whether Russia was to blame, President Vladimir Putin told the BBC: “Get to the bottom of things there, then we’ll discuss this”.