MOSCOW (AP) — The gulf between Russia and Britain widened on Friday as they cranked up pressure over a nerve agent attack and a suspected murder in Britain that have deepened Western worries about alleged Russian meddling abroad.

Britain’s foreign secretary accused Russian President Vladimir Putin of personally ordering the poisoning of ex-Russian spy Sergei Skripal and his daughter, Yulia, describing it as the most brazen such move since World War II.

Putin’s spokesman denounced the claim as “shocking and inexcusable.”

As relations between the two nations sank to a new post-Cold War low, nearly two dozen Russian diplomats in London were packing their bags to leave Tuesday after an expulsion order from Britain. British diplomats in Moscow were bracing for a retaliatory order from the Kremlin and were just waiting to be told who had to leave and when.

Geopolitical tensions have been mounting since the poisoning of the Skripals in the English city of Salisbury on March 4, in what Western powers see as the latest sign of increasingly aggressive Russian interference in foreign countries. The tensions threaten to overshadow Putin’s expected re-election Sunday for another six-year presidential term.

But that’s not all.

New concerns surfaced Friday about the death this week of a London-based Russian businessman, Nikolai Glushkov, found dead at his south London home on Monday. British police said Friday that he died from compression to the neck and opened a murder investigation.

Russia also suspects foul play in Glushkov’s death and opened its own inquiry Friday. Russia’s top agency for major crimes was also investigating the attack on Yulia Skripal, who is a Russian citizen. Her father has British citizenship. Both are in critical condition.

British police said there is no apparent link to the attack on Glushkov and the poisoning of the Skripals.

But to the West, they are raising similar concerns.

While Britain has accused the Russian state of ordering the poisoning of the Skripals, Foreign Secretary Boris Johnson took it a step further Friday and said it’s “overwhelmingly likely” that Putin himself ordered the attack.

Top EU diplomats were expected to discuss next steps at a meeting Monday, with some calling for a boycott of the upcoming World Cup in Russia. British Prime Minister Theresa May is seeking a global coalition of countries to punish Moscow, and the U.S., France and Germany have already lined up against Russia over the Skripal attack.

Britain is expelling 23 Russian diplomats and taking other steps against Russian interests as the two nations’ relations plummet.

“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.

Putin spokesman Dmitry Peskov was quoted by Russian news agencies as calling Johnson’s statement a “shocking and inexcusable breach of diplomatic propriety.” Peskov reiterated Russian denials of involvement in the attack on the Skripals.

“We have never encountered this level of discussion on the global stage,” Peskov told reporters.

Russia ordered a halt to high-level meetings with the U.K. and prepared Friday to expel British diplomats.

Lithuanian Foreign Minister Linas Linkevicius told The Associated Press that the Salisbury attack was a direct challenge to Europe. He said Russia’s recent provocations need a tough response, including action against Russian oligarchs with questionable ties who have used London as a safe haven.

The source of the nerve agent — which Britain says is Soviet-made Novichok — is unclear, as is the way it was administered.

Russia has demanded that Britain share samples collected by investigators.

Russia’s envoy to the Organization for the Prohibition of Chemical Weapons told The AP that his country has no stocks of the Novichok group of nerve agents, insisting that Soviet-era research into the agents was totally dismantled before Russia joined the organization.

Ambassador Alexander Shulgin also sought to shift possible blame, saying Western special agents spirited Russian chemical weapons experts out of the country in the 1990s and work continued on their research.

He said even the name Novichok was a “Western invention” and that Russia never gave it a name.

Hamish de Bretton-Gordon, former commander of the British Army’s chemical and biological weapons regiment, called the claim that U.S. or British agents could have developed Novichok “complete hogwash.”

Speaking to the AP, he called it unlikely that some of the nerve agent could have gone missing in the years after the 1991 collapse of the Soviet Union. He also cast doubt on the possibility that the nerve agent was sent through the mail or was placed in luggage that Skripal’s daughter brought with her from Russia to Britain.

An 83-year-old Russian whistleblower who helped develop Novichok told the AP on Friday that no other country could have used that particular nerve agent to poison a former spy.

Vil Mirzayanov, who now lives in New Jersey, said that if the substance is Novichok, as Britain claims, it’s “100 percent” clear it came from Russia.

Mirzayanov revealed details of Russia’s chemical weapons in the 1990s because he said he was afraid of their impact.

While many British politicians have backed the government in blaming Moscow for the nerve agent attack, the U.K.’s main opposition leader has cautioned against a rush to judgment. Labour Party leader Jeremy Corbyn said in the Guardian that it’s possible that “Russian mafia-like groups,” rather than the Russian state, were responsible.

Britain’s Prime Minister Theresa May speaks to members of military as she visits Salisbury southwest England, Thursday March 15, 2018 . may was in Salisbury view the area of Sunday’s suspected nerve agent attack on Russian double agent Sergei Skripal and his daughter Yulia. (Toby Melville, Pool via AP)