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 spokesman 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.

Murder probe launched into Russian exile's death

British police on Friday launched a murder probe into the death of Russian exile Nikolai Glushkov in London, after a post-mortem found he died from "compression to the neck".

"A murder investigation has been launched following the results of a post-mortem into the death of 68-year-old Russian businessman Nikolay Glushkov," London's Metropolitan Police said in a statement. The retired Russian executive was found dead on Monday evening at his home in southwest London, where he had lived for two years.

'A traitor dies immediately'

Putin has barely weighed in on the row, telling a BBC reporter this week: "Sort things out from your side and then we will discuss this with you."

Russia insists it had no motive to target Skripal with what Britain says was a highly potent Soviet-designed nerve agent called Novichok, in the first such attack in Europe since World War II.

Many Russians remain sceptical that the state was involved in the attack and some analysts didn't rule out the involvement of ordinary criminals or rogue agents.

The official military newspaper Krasnaya Zvezda quoted the former head of Russia's GRU military intelligence, Fyodor Ladygin, as denying his officers had been involved.

"We don't care about the fate of a traitor," said Ladygin. "For an intelligence officer, a traitor dies immediately - he absolutely ceases to exist in the memory. For a traitor, oblivion is death."

The attack on the Skripals revived memories of the fate of Alexander Litvinenko, a Russian dissident who died of Polonium poisoning in a 2006 attack in the UK that London blamed on Moscow.

The leaders of Britain, France, Germany and the United States said Thursday there was "no plausible alternative explanation" for the use of the Soviet-designed nerve agent.

In a joint statement, they demanded Moscow "address all questions" related to the attack against Skripal, which they said amounted to a "breach of international law".

EU president Donald Tusk said Friday that the bloc would discuss the attack at a Brussels summit next week, adding it would send a "clear message". Skripal had taken his daughter, who was on a visit from Moscow, out for lunch before they both collapsed on a bench on March 4.

'We don't want Cold War'

Vil Mirzayanov, a Soviet-era chemist who helped create Novichok but later revealed the existence of Moscow's classified programme, said terrorists could not produce it. "To create its components one needs powerful labs and very experienced personnel which only exist in several countries," the now US-based whistleblower told opposition newspaper Novaya Gazeta.

Britain on Friday said that it had invited the chemical weapons watchdog OPCW to come and take a sample of the poison for analysis, under Article 8 of the Chemical Weapons Convention.

May warned more measures could follow against Russia, noting that the US-led NATO alliance and the UN Security Council had discussed the attack, while it was also expected to be on the agenda at a European Union summit next week. NATO Secretary General Jens Stoltenberg said Friday the alliance did not want a return to Cold War hostilities with Russia while expressing support for Britain's stance. "We don't want a new Cold War," he told BBC radio.