British Prime Minister Theresa May said it is "highly likely" that Russia was responsible for the nerve agent poisoning of former spy Sergei Skripal and his daughter Yulia.

Speaking at the House of Commons on Monday, May called the assassination attempt a "reckless and despicable act".

Skripal, 66, and his daughter Yulia, 33, have been in hospital in critical condition since March 4, when they were found unconscious on a bench outside a shopping centre in the southern English city of Salisbury.

"It is now clear that Mr Skripal and his daughter were poisoned with a military grade nerve agent of a type developed by Russia. This is a part of group of nerve agents known as Novichok," May said.

"Either this was a direct act 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."

May added if Russia would not give an explanation of which of the two scenarios had occurred, as well as disclosure of the country's Novichok programme by the end of Tuesday, the UK government would "conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom".

'Bottom of things'

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 and four years later was given refuge in Britain after being exchanged for Russian spies.

Russia said May's allegations were politically motivated and a provocation ahead of Sunday's presidential elections, according to Russian media.

TASS news agency said Russian foreign ministry spokeswoman Maria Zakharova called the allegations "a circus show".

Ahead of May's comments, President Vladimir Putin said the UK should investigate what happened before levelling accusations at Russia.

"You first get to the bottom of things over there, and after that, we can discuss it," Putin was quoted as saying on Monday.

His spokesman, Dmitry Peskov, said of Skripal: "This Russian citizen worked for one of the British intelligence agencies, the incident took place on British territory and, therefore, it has nothing to do with Russia, let alone the Russian leadership."

The Russian embassy in London said it was "outraged" by an "anti-Russian media campaign".

"Current policy of the UK government towards Russia is a very dangerous game played with the British public opinion, which not only sends the investigation upon an unhelpful political track, but also bears the risk of more serious long-term consequences for our relations," a statement said.

Trace contamination

On Sunday, hundreds of patrons of a pub and restaurant in Salisbury were urged to wash their clothes and possessions.

The poisoning has drawn comparisons to the murder of Alexander Litvinenko in 2006. The former Russian spy died three weeks after drinking green tea laced with radioactive polonium-210 at London's Millennium Hotel.

Russia denied involvement in his death.

A British inquiry into the 43-year-old's death said Moscow had "probably" ordered the poisoning of Litvinenko, who lived in exile in Britain.