(CNN) Russian President Vladimir Putin on Sunday sloughed off the notion that Russia was behind the poisoning of a former Russian spy and his daughter, saying "any sensible person would understand that this is delirium and nonsense, it is unthinkable that we would do such a thing."

"The first thing that came to my mind: If it was military grade agent, they would have died on the spot, obviously," said Putin, speaking at party headquarters in Moscow shortly after he spoke to a crowd in central Moscow. "Russia does not have any such agents, we destroyed it all."

He made the remarks hours after UK Foreign Secretary Boris Johnson said the United Kingdom has evidence Russia has been creating and stockpiling the Soviet-era "military-grade" nerve agent Novichok over the last decade.

Sergei Skripal and his daughter Yulia remain in critical condition after being attacked March 4 in the English city of Salisbury with a nerve agent the UK identified as Novichok.

The assault stoked British rage over what UK Prime Minister Theresa May called "a brazen attempt to murder innocent civilians on our soil."

"We actually had evidence within the last 10 years that Russia has not only been investigating the delivery of nerve agents for the purposes of assassination but also has been creating and stockpiling Novichok," said Johnson, interviewed Sunday on BBC's "The Andrew Marr Show."

Johnson said technical experts from the Organization for the Prohibition of Chemical Weapons will travel to Britain on Monday and will test samples of the substance used in the Salisbury poisoning in an international laboratory. The OPCW, based in The Hague, implements the Chemical Weapons Convention.

Novichok, which means "newcomer" in Russian, was developed in secret by the Soviet Union during the Cold War in the 1980s, as a means of countering US chemical weapons defenses.

Its existence remained secret until the mid-'90s, when information regarding its production was revealed as part of a deliberate leak by disgruntled Soviet scientist and whistle-blower Vil Mirzayanov. Even today, no country outside Russia is known to have developed the substance.

Russian denials

Earlier Sunday, Russian Ambassador to the EU Vladimir Chizhov suggested Porton Down, the UK's chemical weapons research facility, might be behind the March 4 attack, although he said he has no evidence of that.

"When you have a nerve agent or whatever you check it against certain samples that you retain in your laboratory," Chizhov told the BBC on Sunday. "And Porton Down, as we now all know, is the largest military facility in the United Kingdom that has been dealing with chemical weapons research and is actually only 8 miles from Salisbury."

But when asked whether Porton Down was responsible for the attack on the Skripals, Chizhov said, "I don't know, I don't have evidence of anything being used."

Chizhov said Britain's case is "based on assumptions, based on suspicions, fueled by emotions" and said Skripal is a traitor and "almost forgotten" back home.

The British government website says Porton Down produces "very small quantities of chemical and biological agents" so scientists can "develop effective medical countermeasures." It says they are stored securely and disposed of safely when no longer required.

'Two plausible explanations'

Last week, May said the use of the "military grade" nerve agent was an "indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk. And we will not tolerate such a brazen attempt to murder innocent civilians on our soil."

Johnson was asked about one point May told lawmakers: that there were only "two plausible explanations" for the attack.

"Either this was a direct act by the Russian state against our country. Or the Russian government lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others," May said.

Johnson said Sunday that Russia's reaction to the incident "was not the response of a country that really believes itself to be innocent. This is not the response of a country that really wants to engage in getting to the bottom of the matter."

"We gave the Russians every opportunity to come up with an alternative hypothesis, such as the one that you have just described, and they haven't. Their response has been a sort of mixture of smug sarcasm and denial, obfuscation and delay."

On Friday, Johnson said it was "overwhelmingly likely" that Russian President Vladimir Putin personally gave the order to use the nerve agent to attack Skripal. On Sunday, Johnson wrote that Britain is not alone in facing Russia's "reckless behavior."

In an op-ed Sunday for The Sun, Johnson said the Salisbury poisoning was the "latest brazen defiance of international rules" citing the Crimea annexation, cyberattacks in Ukraine, and Russia's interference in European elections.

Tit for tat responses

Russia's Foreign Ministry ordered the expulsion of 23 British diplomats from Russia on Saturday in response to Britain's decision to expel Russian envoys in connection with the Skripal case.

The ministry also declared it was closing the British Consulate General in St. Petersburg and the British Council in Russia, in a step beyond the measures taken by Britain. The British Council is a cultural institute with artistic, language and educational programs.

The UK diplomats have a week to leave, the Russian Foreign Ministry said, adding that its actions came "in response to the provocative actions of the British side and groundless accusations" against Russia over the Salisbury attack.

"We, the leaders of France, Germany, the United States and the United Kingdom, abhor the attack that took place against Sergei and Yulia Skripal in Salisbury". Full statement: https://t.co/Om1uqm9K58 — UK Prime Minister (@10DowningStreet) March 15, 2018

France, Germany and the United States joined the United Kingdom on Thursday in condemning the attack.