Boris Johnson has said it is “overwhelmingly likely” that the Russian president, Vladimir Putin, personally took the decision to use a nerve agent to attempt to kill the former double agent Sergei Skripal on UK soil.

Kremlin says accusing Putin of ordering spy attack is 'unforgivable' Read more

The foreign secretary said Britain was taking retaliatory action against “Putin’s Kremlin” and not the Russian people, as he visited the Battle of Britain Bunker museum in Uxbridge with his Polish counterpart.

“We think it overwhelmingly likely that it was his [Putin’s] 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. That is why we are at odds with Russia,” Johnson said.

Putin’s spokesman, Dmitry Peskov, said Johnson’s personal accusation against the Russian president was 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 the Tass news agency.

On Wednesday Theresa May announced new sanctions against Moscow, including the expulsion of 23 Russian diplomats who the government said were undisclosed intelligence agents. Peskov said Moscow’s response “will be coming shortly” and said the accusations against Russia “not only violate international law but also run against common sense”.

Meanwhile, the world body responsible for policing chemical weapons disclosed on Friday that Russia had not declared information about the existence of the novichok group of nerve agents.

“There is no record of the novichok group of nerve agents having been declared by a state party,” the Organisation for the Prohibition of Chemical Weapons (OPCW) said.

International pressure on Russia has been growing, with Britain, the US, France and Germany on Thursday jointly condemning an “assault on UK sovereignty”. May said the four countries were “very clear in attributing this act to Russia” and the statement showed that “allies are standing alongside us and saying this is part of a pattern of activity that we have seen from Russia”.

The defence secretary, Gavin Williamson, used a speech in Bristol on Thursday to accuse Russia of “ripping up the international rulebook” through actions aimed at subverting other countries. Russia should “go away and shut up”, he said when asked how the Kremlin should respond to the expulsion of 23 of its diplomats.

The US treasury announced new sanctions against Russian individuals and entities – including the FSB and GRU intelligence agencies – as well as the Internet Research Agency in St Petersburg.

Poland’s foreign minister, Jacek Czaputowicz, said on Friday that his country backed the tough stance. “As far as sanctions are concerned Poland would support such action,” he said during the museum visit with Johnson.

“I think the international community has to mobilise around this case in order to demonstrate to Russia decisiveness and there is no acceptance to the breach of international law.”

The OPCW’s decision to disclose that Russia did not declare information about the existence of the novichok group of nerve agents will be seen as a boost for the British position. Signatories to the convention banning chemical weapons, including Russia, have a legal obligation to reveal the whole range of chemical weapons in their arsenal.

The UK has asked the OPCW to verify that novichok was used in the attack on Sergei Skripal and his daughter, Yulia, in Salisbury.

The OPCW, which reports to but is not part of the UN and guards its neutrality carefully, confirmed it was cooperating with the UK authorities. It indicated that some action was imminent but did not elaborate.

“As was stated by the UK authorities, the OPCW offered technical assistance for the UK’s investigation and the OPCW expects some action will be taken soon,” it said.

In a long statement it also said: “There is no record of the novichok group of nerve agents having been declared by a state party to the chemical weapons convention. About such chemicals, there is very little information in open literature.”

It said OPCW experts were reviewing scientific and technical information.