London(CNN) UK Foreign Secretary Boris Johnson said Friday it was "overwhelmingly likely" that Russian President Vladimir Putin personally gave the order to use a nerve agent to attack a former double spy in what's the most direct accusation yet against Russia's leader.

Prime Minister Theresa May announced this week that 23 Russian diplomats would be expelled from Britain after concluding it was highly probable that Moscow was behind the poisoning, but she stopped short of pointing the finger directly at Putin.

The ex-spy, Sergei Skripal, and his daughter Yulia remain in a critical condition after the March 4 attack in the English city of Salisbury. UK officials believe they were exposed to a nerve agent known as Novichok that was developed in Russia.

Kremlin spokesman Dmitry Peskov slammed Johnson's remark in a comment to CNN.

"We have said on different levels and occasions that Russia has nothing to do with this story," he said. "Any reference or mentioning of our President is nothing else but shocking and unpardonable diplomatic misconduct."

Russia has repeatedly dismissed the UK's accusations as "unfounded" and warned it would retaliate over the expulsion of its diplomats.

Speaking alongside his Polish counterpart at the Battle of Britain Bunker in outer London, Johnson said the UK and its allies were waiting for a serious response from Russia about the nerve agent attack.

"The quarrel of the UK government is not with Russian people -- it is not with Russians living here in this country. We have nothing against the Russians themselves. There is to be no Russophobia as a result of what has happened," he said.

"Our quarrel is with Putin's Kremlin and with his decision -- and we think it is overwhelmingly likely that it was his decision -- to direct the use of a nerve agent on the streets of the UK, on the streets of Europe, for the first time since the Second World War."

Polish Foreign Minister Jacek Czaputowicz expressed his "full solidarity" with the UK, adding he was prepared to work with other European Union and NATO countries to protest against Russia's behavior.

"We condemn this unprecedented attack by Russia on the territory of the United Kingdom. This use of chemical weapons is a clear violation of the international law," he said.

Police official Kier Pritchard and British Prime Minister Theresa May view the crime scene in Salisbury.

Russia launches proceedings

Russia has insisted it is ready to cooperate in investigating the attack in Salisbury if Britain reciprocates by sharing the evidence it holds -- and has dismissed accusations against it as "propaganda" by the UK government.

Vassily Nebenzia, Moscow's ambassador to the United Nations, even suggested at an emergency session of the UN Security Council that the UK might have been responsible for the attack in an attempt to smear Russia.

On Friday, Russia's Investigative Committee said it had launched its own criminal proceedings in connection with the "attempted murder of a Russian citizen, Yulia Skripal" in Salisbury and what it called the "murder" of Nikolai Glushkov in London.

Glushkov, a Russian exile who had links to compatriots who died in mysterious circumstances in the UK, was found dead in his London home this week.

London's Metropolitan Police said Friday it had launched a murder investigation into Glushkov's death following the results of a post-mortem exam, which gave the cause of death as "compression to the neck."

Counterterrorism police are leading the investigation "because of the associations Mr. Glushkov is believed to have had," a police statement said. "At this stage there is nothing to suggest any link to the attempted murders in Salisbury, nor any evidence that he was poisoned."

In an online post, the Russian committee said the attempt on Yulia Skripal's life was "committed in a dangerous manner endangering other people, in Salisbury."

The investigation will be carried out in accordance with "Russian and international law," it said, adding that "investigators are ready to work together with competent authorities in Great Britain."

The statement made no mention of Sergei Skripal.

'An assault on UK sovereignty'

On Thursday, the United States issued a joint statement with France, Germany and the UK condemning the nerve agent attack as "an assault on UK sovereignty" and saying there was "no plausible alternative explanation" than that Russia was responsible.

Meanwhile, British Defense Secretary Gavin Williamson said Moscow had made a "deliberate political decision" to poison Skripal. He accused Russia of "ripping up the international rulebook" and "attempting to "subvert, undermine and influence" countries around the world.

"Russia should shut up and go away," Williamson said. "It's often described as a cool war that we are entering -- I would say it is feeling exceptionally chilly at the moment."

UK-Russia relations have been fractious ever since the assassination of another former Russian spy, Alexander Litvinenko, in 2006.

A UK inquiry found that two Russian agents poisoned Litvinenko at a London hotel bar in 2006 by spiking his tea with highly radioactive polonium-210, and that Putin "probably approved" Litvinenko's killing. The Kremlin has always denied the accusation.

Sergei Skripal was sentenced to 13 years in prison in 2006 for spying for Britain, according to Russian state media accounts of the closed hearing.

Russian court officials at the time said he'd received at least $100,000 for his work for MI6, the British intelligence service. He was granted refuge in the UK after a high-profile spy exchange between the United States and Russia in 2010.