London: Russian President Vladimir Putin is "overwhelmingly likely" to have ordered the nerve agent attack on an ex-spy and his daughter, British Foreign Secretary Boris Johnson has said.

Johnson said "our quarrel is with Putin's Kremlin, and with his decision" over the Salisbury incident, BBC reported on Friday.

Russia denies involvement and said the accusations against Putin were "shocking and unforgivable". Meanwhile, the head of Nato told the BBC that Russia has underestimated the "resolve and unity" of the UK's allies.

Speaking during a visit to a west London military bunker with the Polish foreign minister, Mr Johnson said the UK's "quarrel is with Putin's Kremlin".

"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," he said.

Russian ex-spy Sergei Skripal, 66, and his daughter Yulia Skripal, 33, remain critically ill in hospital, after they were found unconscious on a bench in the Wiltshire city on 4 March.

The UK government says they were poisoned with a nerve agent of a type developed by Russia called Novichok, and Prime Minister Theresa May said she believed Moscow was "culpable".

May has said the UK will expel 23 Russian diplomats as part of a "full and robust" response -- prompting Russia's Foreign Minister Sergei Lavrov to say it will "certainly" expel British diplomats in response.

Russia's investigative committee said it had opened a criminal case investigation into the "attempted murder" of Skripal and "the murder" of Nikolai Glushkov, a Russian exile who was found dead in his home in London on Monday.

According to Russian news agency Tass, the Russian ambassador to Britain, Alexander Yakovenko, said the UK claimed the nerve agent used was A-234, but this has not been confirmed.

A Downing Street spokesman said the Organisation for the Prohibition of Chemical Weapons (OPCW) has been invited to come to the UK to take a sample of the nerve agent -- that process is expected to begin "imminently".

According to a latest report in the Guardian, The Kremlin has called direct international accusations that Vladimir Putin ordered the Salisbury nerve agent attack “shocking and unforgivable”.

The remarks come amid rising tensions between London and Moscow. The Russian foreign minister, Sergei Lavrov, and the Kremlin spokesman Dmitry Peskov have said repeatedly that Moscow would retaliate soon for the expulsion of 23 Russian diplomats by Theresa May.

Speaking to reporters on Friday, Peskov repeated denials that Russia had ordered the attack on Sergei Skripal, a former Russian double agent, and his daughter, Yulia, in Salisbury. “Any references to our president is nothing other than shocking and unforgivable from the point of view of diplomatic behaviour,” he said.

IANS