If the Putin government fails to explain what it knew of the attempt on Sergei Skripal's life, the UK is likely to impose tougher sanctions.

British Prime Minister Theresa May has given Moscow until late Tuesday to provide a “credible response” to her accusation that Russia is behind the poisoning of former double agent Sergei Skripal on March 4.

In an explosive House of Commons statement, May revealed it was “highly likely” Russia was involved based on evidence provided by UK’s main intelligence services.

Biological experts at the British military’s Porton Down chemical weapons research centre ascertained that the poison used in the attack in Salisbury, England — which has left Skripal and his daughter Yulia fighting for life — was a military-grade nerve agent from a family of chemical weapons called Novichok, which were developed by the Soviet Union during the Cold War.

“Based on Russia’s record of doing state-sponsored assassinations and our assessment that Russia views some defectors as legitimate targets for assassinations, the government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal,” 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.”

If the credible response is not forthcoming, May plans to impose “much more extensive” sanctions against President Vladimir Putin’s government than are already currently in place.

“Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom.”

This follows Foreign Minister Boris Johnson’s threat last week that Britain will “respond robustly” if the Kremlin is found to be involved.

He said there were “echoes” of the murder of former Russian spy Alexander Litvinenko in London in 2006, which a UK public inquiry deduced was “probably” aproved by Putin.

“No attempt to take innocent life on UK soil will go either unsanctioned or unpunished,” Johnson added.

Russia says UK is inventing fairytales

Moscow has denied any involvement in the assassination attempt, with Russia’s foreign ministry shrugging off May’s intervention and the ultimatum, as a “circus show in the British parliament”.

“The conclusion is obvious: another information and political campaign based on provocation,” said Maria Zakharova, a Russian foreign ministry spokeswoman.

According to BBC Monitoring, which translates foreign media, Zakharova also implied that Russia has been falsely implicated in a string of deaths of Russian identities in Britain, and this is just another.

“Before inventing any more fairy tales, let someone in the Kingdom say what was the outcome of the cases of Litvinenko, Berezovsky and Perepilichny and many others who mysteriously died on British soil,” she reportedly said.