The poisoning of former Russian spy Sergei Skripal in Salisbury, England has metastasized into the most serious confrontation between Russia and the West since the annexation of Crimea four years ago.

"This is a state-sponsored act of terrorism using chemical weapons by Russia — I don't think there could be anything more grave," said Bill Browder, the London-based businessman turned human rights crusader, who has emerged as one of Russian President Vladimir Putin's fiercest critics.

British Prime Minister Theresa May says the attack was either a 'direct act' of the Russian state, or the Russian government lost control of the nerve agent and allowed it to fall into someone else's hands. (Simon Dawson/Reuters)

On Monday, British Prime Minister Theresa May confirmed what many in Britain had already made up their minds about — that it is "highly likely" Russia tried to assassinate Skripal, a one time Russian-British double agent.

The attack has left both Skripal, 66, and his 33-year -old daughter Yulia in critical condition in hospital.

May told the British Parliament that the nerve agent used to poison Skripal further affected dozens of other people, including a police officer who investigated the crime scene and who has yet to fully recover.

Strong evidence

The direct and circumstantial evidence supporting Russian government involvement is powerful.

May said British scientists have determined the pathogen was part of the Novichok group of nerve agents, developed by the Soviet Union during the Cold War and still held by Russia.

As forensic police continue their investigation, Russian state media and Kremlin sources deny any involvement in the poisoning. (Neil Hall/EPA-EFE)

She also said the poisoning followed a strikingly similar pattern to the fatal attack on former Russian KGB agent Alexander Litvinenko, who was murdered in London in 2006 after drinking tea laced with a radioactive material also used by Russia.

That incident also contaminated scores of other people who came in contact with the substance.

"They [Russia] have the ability, the willingness and history of doing this," Browder told CBC News in an interview.

But May also says the incident followed a similar pattern to the fatal attack on Alexander Litvinenko, a former Russian KGB agent turned British citizen who was murdered in London in 2006. (Toby Melville/Reuters)

Nonetheless, the usual Kremlin sources and Russian state media robustly deny any involvement in the Skripal poisoning.

Even after May's accusations late Monday, Russian officials repeatedly mocked suggestions about any state-sponsored assassination scheme.

'Circus show'

"This is a circus show in the British Parliament," Maria Zakharova, the official spokesperson for Russia's Foreign Ministry, said Monday night.

Zakharova is well known for her blistering diatribes against Western countries.

"The conclusion is obvious," she said. "[It is] another information and political campaign based on provocation."

Maria Zakharova, a spokesperson for Russia's foreign ministry, says May's evidence is part of a 'fairy tale.' (Maxim Shemetov/Reuters)

Russian state TV and its many talk programs have done a hard sell on the unsupported notion that Skripal was probably killed by the British government, which presumably stole the nerve agent to make it easier to shift the blame to Russia.

The conspiracy theorists claim — without any proof — that the incident feeds into a much bigger NATO plot to whip up international anger against Russia.

Zakharova referred to the evidence May presented as being part of a "fairy tale."

"Let someone in the kingdom tell what the outcome [was] of previous cases of Litvinenko, Berezovsky and Perepilichny and many others who mysteriously died on British soil," she said.

The three men were all Putin critics, and all died under mysterious circumstances in London.

The implication seemed to be that the British government has a history of killing former Russian double agents and then blaming the Kremlin.

Disinformation campaign

The official British government inquiry into Litvinenko's killing suggested it was ordered by figures in the Kremlin, perhaps even by Putin himself.

Supporters of Russian President Vladimir Putin say the allegations are part of an effort to discredit Russia. The country will host the FIFA World Cup in three months' time. Russian media have suggested this is all part of a plot to justify a boycott of the tournament. (Sputnik/Alexei Nikolskyi/Kremlin via Reuters)

Russian media have turned to similar disinformation tactics several times in the past, most notably after Malaysian airlines flight MH17 was shot down in 2014 over eastern Ukraine by pro-Russian separatists using a Russian-made missile.

Despite the convincing work by Dutch investigators who tracked when and where the missile launcher left and then re-entered Russia, the Putin government continues to insist the missile was fired by Ukrainian forces who might have been trying to shoot down Putin's plane.

May said she has summoned the Russian ambassador to explain whether the Russian government was directly responsible for using the Novichok agent, or if it somehow lost track of it.

But Browder says he doubts that.

"It's clear in my mind there is no such thing as a rogue operation," he said. "There would be no group inside the security service that would do this and endure the wrath of Vladimir Putin. They would only do this with his approval."

Robust response

Whatever the answer, May is now facing calls to respond in the toughest way possible. That could include expelling Russian diplomats from London and freezing the assets of rich Russians connected to Putin's regime.

With the FIFA World Cup coming to 11 Russian cities in just three months, Russian media have also been full of speculation that the poisoning could be part of a Western plot to justify a boycott of the tournament.

There's no sign of that — yet.