British Foreign Secretary Boris Johnson said Friday it was “overwhelmingly likely” that Russian President Vladi­mir Putin was behind the poisoning of a Russian former spy, the most direct British accusation against the Russian leader to date.

Kremlin spokesman Dmitry Peskov immediately shot back, saying that bringing up Putin in the context of the case was “shocking and unforgivable in terms of diplomatic behavior.”

Johnson’s comments followed Prime Minister Theresa May’s decision Wednesday to expel 23 Russian diplomats from Britain. Russia confirmed Friday it will expel British diplomats and halt high-level meetings in turn.

“Our quarrel is with Putin’s Kremlin, and with his decision — and we think it overwhelmingly likely that it was his decision — to direct the use of a nerve agent on the streets of the U.K., on the streets of Europe for the first time since the Second World War,” Johnson said during a visit to a museum in London.

The poisoning of 66-year-old Sergei Skripal and his 33-year-old daughter, Yulia, in Salisbury with a nerve agent identified by British authorities as one made only by Russia has thrown the two countries’ relations into a profound crisis.

Aside from confirming it would expel some British diplomats, without giving the number, Russia has been coy about its potential responses.

“The Russian side has made its decisions on tit-for-tat measures, and the British side will be notified of them not in the next few hours, but in the near future,” Russian Foreign Ministry spokesperson Maria Zakharova told Interfax on Friday.

Alexander Gabuev, a Russian foreign policy analyst with the Carnegie Moscow Center, said the delay is likely related to Sunday’s presidential election.

“They’re either saving the response for closer to the big day or want to minimize negative news until after the election,” he said. Because May delivered the British response, Putin will want to deliver Russia’s.

“Simply expelling 23 British diplomats probably won’t be enough,” Gabuev said. “There are other elements to Britain’s reaction, and you need to give a ‘mirror’ response to that as well.”

The Russian government also has been vague about its response to Washington’s expansion of sanctions announced Thursday against Russian individuals believed to have played a role in alleged cyberattacks and attempts to influence the 2016 U.S. presidential election.

Russian Deputy Foreign Minister Sergei Ryabkov said Friday that Moscow would expand its own “black list” of Americans and added that additional measures have not yet been ruled out. “Those [American] politicians are playing with fire,” he said.

In a separate development Friday, London police announced the death of Nikolai Glushkov, a 68-year-old Russian businessman found unresponsive in his London home Monday, is being investigated as a murder. A pathologist report identified the cause of death as “compression to the neck.”

“At this stage, there is nothing to suggest any link to the attempted murders in Salisbury, nor any evidence that [Glushkov] was poisoned,” police said in a statement.

London’s counterterrorism officers are leading the investigation, police said, because of the “associations Mr Glushkov is believed to have had.” He was a close friend of Boris Berezovsky, a Russian dissident who died in mysterious circumstances in 2013.

The Guardian reported that earlier Monday, Glushkov had failed to show up at a London court where he was scheduled to defend himself against charges of stealing from the Russian state airline Aeroflot.

Russia also announced it would be opening an investigation into Glushkov’s death.

In the Salisbury case, Russia has focused its efforts on a campaign of denial and counterclaim in which officials at times have contradicted each other.

On Thursday, Ryabkov claimed that Russia had never developed anything like the alleged nerve agent, identified by the British as Novichok. Shortly after, a Russian lawmaker charged that the United States stole samples while helping to decommission the facility where Novichok was made in the 1990s. Most statements have fallen somewhere in between the two extremes.

Russian Foreign Minister Sergei Lavrov on Friday continued to deny and deflect blame, claiming again that British allegations of Russian involvement in Skripal’s poisoning were groundless and anti-Russian. He wished the Skripals a speedy recovery and said he hopes they can shed light on what happened when they are well.

Lavrov also lashed out at Britain for not providing consular access to Yulia Skripal, who, along with her father, is in critical condition at a Salisbury hospital. Later Friday, Russia’s Investigative Committee announced it was opening a criminal investigation into the attempted murder of the younger Skripal.

The Skripals were found slumped over on a park bench in the cathedral town of Salisbury, located near the famed ruins of Stonehenge. An officer who attempted to revive them remains in the hospital in stable condition. Several areas in the town are still cordoned off as police continue their investigation.

Writing in the Guardian on Friday, opposition Labour Party leader Jeremy Corbyn called for “calm heads” and warned against rushing into a “new Cold War.”

He suggested that the possibility that the Russians had lost control of the dangerous nerve agent — which May floated Monday but has since discounted — could not be excluded.

He referenced the “flawed intelligence and dodgy dossiers” ahead of the 2003 invasion of Iraq. “In my years in parliament I have seen clear thinking in an international crisis overwhelmed by emotion and hasty judgments too many times,” he wrote.

Corbyn also argued that targeting the “oligarchs and their loot would have a far greater impact on Russia’s elite than limited tit-for-tat expulsions.”

London is something of a hub for Russian expats — former dissidents and super-rich oligarchs alike are drawn to its good schoolsand its laws and courts. The wealthiest snap up London’s luxury homes for eye-popping prices.

Anti-corruption advocates say some of that property is bought with questionable funds. A 2017 report by Transparency International linked Russia to about of fifth — or $1.3 billion — of the “suspicious wealth” used to purchase London property.

Analysts say this means Britain could go after Russian interests.

“The Russians have property and children and business in the U.K.,” said James Nixey, a Russia expert at Chatham House, a London-based think tank. “The U.K. has leverage there.”

The United States, France and Germany on Thursday formally backed Britain’s claims that Russia likely was responsible for the attack, calling it the “first offensive use of a nerve agent” in Europe since World War II.

In a statement released by May’s office, May, President Trump, French President Emmanuel Macron and German Chancellor Angela Merkel said they shared the view of British investigators that “there is no plausible alternative explanation” for the attack. They added that “Russia’s failure to address the legitimate request by the U.K. government further underlines its responsibility.”

“It threatens the security of us all,” they added, without spelling out any possible further reprisals.

Adam reported from London.