MOSCOW — 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, in 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. There has been much talk in Moscow of imminent and decisive retaliation, but so far nothing of substance has taken place.

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 has also 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, adding that additional measures have not yet been ruled out. “Those [American] politicians are playing with fire,” he said.

Russian Foreign Minister Sergey Lavrov speaks to Federal Security Service Director Alexander Bortnikov as they attend a security council meeting in the Kremlin in Moscow, Russia, on Thursday. Lavrov said Thursday that Moscow would “certainly” expel some British diplomats in a tit-for-tat response. Mikhail Klimentyev, Sputnik, Kremlin Pool Photo via Associated Press

Russia instead 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 Sergey 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 also 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.”

Undeterred by criticism from across the country and even within his own party, Corbyn did not endorse the British government’s claim that there was “no alternative conclusion” other than that the Russian state was culpable. 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 also 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.

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, the four leaders 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.