British Foreign Secretary Boris Johnson’s allegation that Russian President Vladimir Putin ordered the poisoning of a former double agent in England is “unpardonable diplomatic misconduct,” according to the Kremlin.

"Any reference or mentioning of our president is nothing else but shocking and unpardonable diplomatic misconduct,” Putin spokesman Dmitry Peskov told TASS, a state-run media outlet.

Russia has claimed no involvement in the attempted assassination of a former military intelligence officer convicted of treason for passing secrets to the United Kingdom. British Prime Minister Theresa May has dismissed those denials, and emphasized that former spy Sergei Skripal was stricken by “a military-grade nerve agent” developed during the Cold War by the Soviet Union. Russian officials have decried the charges as a “Russophobic campaign” launched by the West.

Johnson countered that the British have a much narrower complaint. “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 Friday. “There is to be no Russophobia as a result of what has happened.”

Russian officials have suggested that the United Kingdom developed its own version of the nerve agent and poisoned Skripal in order to frame Putin for the crime in the run-up to the March 18 presidential election in Russia.

“[T]he most probable source origin for this chemical are the countries which have since the end of the '90s been carrying out intensive research on these kinds of weapons, including the UK,” Russian Ambassador Vasily Nebenzia told the United Nations Security Council on Wednesday. “If the UK is so firmly convinced this is a [Soviet-era] Novichok gas, then that means that they have the samples of this and they have the formula for this and they are capable of manufacturing it.”

The Skripal incident has led to an escalating diplomatic feud. May expelled Russian diplomats and froze high-level contacts as punishment for what she calls “an unlawful use of force by the Russian State against the United Kingdom.”

Russia has vowed to retaliate.

“[A]ll steps, will, course, be well thought-out and will fully meet our country’s interests,” Peskov said Friday.