LONDON, March 13 (Xinhua) -- A nerve agent, called Novichok that has led to a former Russian spy and his daughter fighting for their lives in Britain, had not been used in Europe since World War II, British Foreign Secretary Boris Johnson said Tuesday.

Sergei Skripal, aged 66 and his 33-year-old daughter Yulia Skripal, a Russian citizen, remain in a critical condition in hospital after being exposed to the nerve agent in the southern England city of Salisbury on March 4.

Hundreds of officers continued Tuesday to work around the clock gathering evidence to identify those responsible for the attempted murders of the pair, in what anti-terror cops in London described as a targeted attack.

Johnson issued a statement Tuesday through the Foreign Office in London in which he described what happened in Salisbury in Wiltshire County as a "brazen attempt to murder innocent people on British soil.

"It's overwhelmingly likely, or highly likely that the Russian state was involved," said Johnson.

A statement from the Russian Embassy said the British Ambassador to Russia, Laurence Bristow, was summoned Tuesday to the Russian Foreign Ministry, where First Deputy Foreign Minister Vladimir Titov strongly protested the evidence-free accusations by British authorities of Russia's alleged involvement in the poisoning case.

"It was stated that the actions of the UK authorities are a clear provocation and that the Russian Federation was not involved in the incident that took place in Salisbury on March 4, 2018," the statement added.

It continued: "From the Russian side, it was emphasised that Moscow will not respond to London's ultimatum until it receives samples of the chemical substance to which the UK investigators are referring and until the UK demonstrates compliance with the Chemical Weapons Convention that stipulates a joint investigation into the incident, for which Moscow is ready.

"Without that, there can be no sense in any statements from London. The incident appears to be yet another crooked attempt by the UK authorities to discredit Russia. Any threat to take 'punitive' measures against Russia will meet with a response. The British side should be aware of that."

In his statement, Johnson added: "We've given the Russians until midnight to explain how the Novichok could have come to be on the streets of Britain. We cannot exclude that they have an explanation and we will want a full disclosure to the chemical weapons watchdog in the Hague. If not, there is a package of measures that we would use."

Prime Minister Theresa May is expected to make an announcement to MPs in the House of Commons on Wednesday in which she may announce any proposed sanctions or measures against Russia.

Ambassador Alexander Shulgin, Permanent Representative of the Russian Federation to OPCW, made a statement about the Salisbury incident to OPCW's executive council Tuesday describing Britain's unfounded accusations of Russian involvement as "absolutely unacceptable".

Shulgin said: "Our British colleagues should recall that Russia and the United Kingdom are members of the OPCW which is one of the most successful and effective disarmament and non-proliferation mechanisms.

"We call upon them to abandon the language of ultimatums and threats and return to the legal framework of the chemical convention, which makes it possible to resolve this kind of situation," Shulgin said.