Moscow - Britain on Friday provoked Russia's wrath by directly implicating Vladimir Putin in the nerve agent attack on an ex-double agent, with the Kremlin saying the claims were "shocking and unforgivable".
The war of words between Moscow and London over the poisoning of a former Russian spy escalated as Britain's outspoken Foreign Secretary Boris Johnson said his government's "quarrel" was with Putin rather than the Russian people.
"We think it 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," Johnson said in London.
Putin's spokesperson Dmitry Peskov responded saying Johnson's claims violated all rules of diplomatic protocol.
Linking Putin to the attack on Sergei Skripal, who moved to Britain in a 2010 spy swap, "is nothing but shocking and unforgivable behaviour from the point of view of diplomacy," Peskov told Russian news agencies.
The crisis has unravelled in the thick of Russia's presidential campaign, with Putin expected to win a fourth Kremlin term on Sunday.
In a rare joint statement, the leaders of Britain, France, Germany and the United States on Thursday condemned the attack on former double agent Skripal and his daughter Yulia - both in a critical condition in hospital - as an "assault on UK sovereignty".
Moscow opens probes
The Kremlin has vehemently denied it had a hand in the poisoning of its former spy in the English cathedral city of Salisbury early this month.
Britain's key allies have closed ranks against Putin after British Prime Minister Theresa May announced the expulsion of 23 Russian diplomats and suspended high-level contacts, among other measures.
On Friday Russia said it could hit back at Britain at "any minute" with its own raft of punitive measures.
Earlier this week the Kremlin indicated it would expel British diplomats in response to London's move as well as adopt other measures that would "most suit Moscow's interests."
"All the steps will be well thought out," Peskov said on Friday.
Meanwhile the Investigative Committee, which reports to Putin, opened a probe into the "attempted premeditated murder" of Skripal's daughter, a Russian national, which it said had been "carried out in a way that was dangerous to the public."
At the same time a separate probe was opened into the mysterious death of Nikolai Glushkov, a Russian exile who was found dead at his London home this week.
London's Metropolitan Police later announced their own murder investigation, saying Glushkov he had suffered compression of the neck.
