The United Kingdom claimed on Sunday that Russia has been actively researching nerve agents for a decade to use them for assassinations. Foreign Secretary Boris Johnson claimed to have evidence that Russia, for over 10 years, stockpiled the nerve agent that was used to attack a former spy and his daughter on March 4.

Moscow’s reaction to the incident “was not the response of a country that really believes itself to be innocent”, Johnson said on the BBC’s The Andrew Marr Show.

In Moscow on Sunday, Russian President Vladimir Putin called it “total rubbish, drivel and nonsense” that someone in Russia would carry out such “antics” so close to Russia’s presidential election and the football World Cup that starts in June in Russia. He called the March 4 attack a “tragedy”, The New York Times reported.

“We destroyed all of our chemical weapons under the control of international observers,” Putin said soon after winning Sunday’s election. “Moreover, we were the first to do this, unlike some of our partners who promised to do this but have yet, unfortunately, to keep their pledge.”

Former Russian double agent Sergei Skripal and his daughter Yulia were found unconscious on a bench outside a shopping centre in Salisbury on March 4. Both of them, and a police officer who first responded to them, are said to be in a serious condition in hospital.

Police said the substance that rendered them critically ill was a nerve agent. The UK has accused Russia of being responsible for the attack and said Russian President Vladimir Putin may have given the order for the attack.

Later, the UK expelled 23 Russian diplomats after Moscow refused to meet Prime Minister Theresa May’s ultimatum to give information about the attack. Russia, in response, ordered an equal number of UK diplomats to leave the country within a week.