BRITISH Prime Minister Theresa May has accused Russia of involvement in the nerve agent attack on a double-agent Russian spy, sparking a diplomatic row and dramatically raising tensions with Russia ahead of its presidential election on Sunday.

Saying Moscow’s involvement was “highly likely’’, the PM threatened harsher sanctions and other measures against Russia, a move that would raise the expectation that allies such as Australia would follow suit.

Mrs May revealed the chemical weapon used on former Russian spy Sergei Skripal and his daughter Yulia on May 4 was a military-grade nerve agent known as Novichok, first developed by the then-Soviet Union in the 1970s.

She pinned the blame for the attack — which saw 21 people taken to hospital, left the Skripals in a clinging to life, and seriously injured a police officer — on Russia, and gave Moscow a 24-deadline to explain its actions.

“Either this was a direct act by the Russian State against our country,’’ Mrs May told the House of Commons.

“Or the Russian government lost control of this potentially catastrophically-damaging nerve agent and allowed it to get into the hands of others.

“This afternoon the Foreign Secretary (Boris Johnson) has summoned the Russian Ambassador to the Foreign and Commonwealth Office and asked him to explain which of these two possibilities it is — and therefore to account for how this Russian-produced nerve agent could have been deployed in Salisbury against Mr Skripal and his daughter.’’

With Moscow heading to the polls on Sunday in an election all-but-guaranteed to return President Vladimir Putin for another six-year term, Mrs May attacked the Russian regime, accusing it of a string of hostile acts.

“This action has happened against a backdrop of a well-established pattern of Russian State aggression,’’ she said.

“Russia’s illegal annexation of Crimea was the first time since the Second World War that one sovereign nation has forcibly taken territory from another in Europe.

“Russia has fomented conflict in the Donbas, repeatedly violated the national airspace of several European countries, and mounted a sustained campaign of cyber espionage and disruption. This has included meddling in elections, and hacking the Danish Ministry of Defence and the Bundestag, among many others.

“During his recent State of the Union address, President Putin showed video graphics of missile launches, flight trajectories and explosions, including the modelling of attacks on the United States with a series of warheads impacting in Florida.’’

She said the Russian Parliament had also in 2006 given legal sanction to the extrajudicial killing of terrorists and dissidents outside of Russian territory.

Mrs May also raised the assassination by radioactive material of former spy Alexander Litvinenko who was murdered in an apparent state-sponsored hit in London in 2006.

“Following Mr Litvinenko’s death we expelled Russian diplomats, suspended security co-operation, broke off bilateral plans on visas, froze the assets of the suspects and put them on international extradition lists. And these measures remain in place,’’ Mrs May said.

She name-dropped NATO to remind Russia of the power of the military alliance which included most of Europe and the United States.

“We have led the way in securing tough sanctions against the Russian economy,’’ she said.

“And we have at all stages worked closely with our allies and we will continue to do so.

“We must now stand ready to take much more extensive measures.’’

Mrs May said the UK’s Defence Science and Technology laboratory at Porton Down had identified the weapon used on the Skripals as among a group of nerve agents known as Novichok.

“Based on the positive identification of this chemical agent by world-leading experts … our knowledge that Russia has previously produced this agent and would still be capable of doing so; Russia’s record of conducting state-sponsored assassinations; and our assessment that Russia views some defectors as legitimate targets for assassinations; the Government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal,’’ she said.

The Government will consider Russia’s response on Wednesday.

“Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian State against the United Kingdom,’’ Mrs May said.

“This attempted murder using a weapons-grade nerve agent in a British town was not just a crime against the Skripals.

“It was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk.

“And we will not tolerate such a brazen attempt to murder innocent civilians on our soil.’’

What is Novichok?

A new group of nerve agents developed by the former Soviet Union in the 1970s and 1980s, it is considered many times more lethal than VX, the chemical used to assassinate Kim Jong-nam, the half-brother of North Korean Leader Kim Jong-un, at a Malaysian airport in 2016.

It can be dispersed as a fine powder rather than a gas, and is usually absorbed through inhalation.

Fast-acting, it works as other nerve agents do, blocking transmitters to the brain which leads to death by asphyxiations the victim is unable to breathe.