Boris Johnson, Britain's foreign minister, has said it is "highly likely" that Russian President Vladimir Putin himself ordered the nerve-agent attack on a former Russian agent on UK soil.

In a response, Russia has called the allegations "shocking and inexcusable" and a breach of diplomatic rules of decent behaviour.

Johnson said the attack on former Russian double agent Sergei Skripal and his daughter with a military-grade nerve toxin was most likely ordered by Putin himself.

"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 UK, on the streets of Europe for the first time since the second world war," Johnson said.

"We have nothing against the Russians themselves. There is to be no Russophobia as a result of what is happening."

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Theresa May, the British prime minister, expelled 23 Russian diplomats after she accused Russia of being behind the incident.

Responding to the accusations and expulsions by Britain, Kremlin spokesman Dmitry Peskov called Johnson's remarks "shocking and unforgivable".

"Any reference or mention of our president in this regard is a shocking and unforgivable breach of diplomatic rules of decent behaviour," Peskov said according to Russian press agency TASS.

The Kremlin is also preparing retaliatory measures against Britain.

"You can expect it any minute," Peskov said.

Possible retaliation

What these countermeasures will entail exactly is unknown yet, but Russia said earlier it would expel British diplomats.

Moscow has denied any involvement in the attempted assassination of Skripal, a former Russian spy turned double agent for the UK.

Skripal betrayed dozens of Russian agents to British intelligence before his arrest in Moscow in 2004.

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He was sentenced to 13 years in prison in 2006 and four years later was given refuge in Britain after being exchanged for Russian spies.

Skripal, 66, and his daughter Yulia, 33, have been in hospital in critical condition since March 4, when they were found unconscious on a bench outside a shopping centre in the southern English city of Salisbury.

According to British experts, the toxin used in the attack was part of a group of nerve agents known as Novichok, which was developed in the former Soviet Union.

The poisoning has drawn comparisons to the murder of Alexander Litvinenko in 2006.

The former Russian spy died three weeks after drinking green tea laced with radioactive polonium-210 at London's Millennium Hotel.

Russia denied involvement in his death.

A British inquiry into the 43-year-old's death said Moscow had "probably" ordered the poisoning of Litvinenko, who lived in exile in Britain.