British Prime Minister Theresa May has blamed Moscow for the poisoning of a former Russian-British double agent in the UK and given Russia 24 hours to explain the incident, officially escalating a crisis point in bilateral relations.

Speaking in the House of Commons on Monday, UK time, Ms May said “It is now clear that Mr Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia. The government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal.”

Russian Foreign Minister Sergei Lavrov on Tuesday insisted that Moscow is not to blame for the poisoning and said it was ready to cooperate with London.

"Russia is not guilty. Russia is ready to cooperate according to the Chemical Weapons Convention, if Britain takes the trouble and condescends to carry out its international obligations according to the same document," Lavrov said at a press conference.

Russian security expert Dr Alexey Muraviev from Curtin University said that while London and Moscow have a long shared history of espionage incidents and spy intrigue, this one was different because innocent civilians were also hurt.

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Mr Skripal, a former Russian military intelligence official who was recruited as a double agent for the British in the 1990s, was admitted to hospital along with his daughter after being exposed to the nerve agent on 4 March, in Salisbury, a town south-west of London. They remain in a critical condition. By the end of last week, 21 people had been treated for exposure to the poison.

“Technically this can be classified as an act of terror,” Dr Muraviev told SBS News. “When you start using poisons or toxins on foreign soil … that’s what really makes it different, that makes it stand out.”

Was it definitely the Russians?

The UK says the nerve agent used in the attack was from a chemical weapon substance group of highly toxic nerve agents known as Novichok, developed in Russia in the 1970s and 1980s, Dr Matthew Sussex, a Russia foreign policy and security expert from the Australian National University told SBS News.

“The Russians are the only ones that have these weapons,” he said. “It is a military grade chemical weapon of war. It’s not like feeding someone Ratsak (rat poison). It’s unambiguous that it was state-sponsored: either they’ve got loose stockpiles and rogue people running around or it came from the Russian government.”

Then there’s the identity of the target. Skripal was arrested in 2004 and confessed to Russian authorities that he had been working for MI6, the UK's Secret Intelligence Service. He pleaded guilty and was given a shorter sentence of 13 years, but surprisingly was pardoned by the former Russian President Dmitry Medvedev in 2010, and sent to Britain in a spy swap deal after 10 Russian operatives were captured in the US.

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“Technically he was pardoned before the spy swap,” Dr Muraviev said, “But for Russian intelligence there is no such thing as a pardon, a traitor is a traitor. If you betray your comrades in arms, you are potentially their target for life.

“Skripal sold not just Russian secrets but also his comrades, other agents working abroad for Russia. … The Russians never forgive something like that.”

Dr Sussex said it was likely Skripal's attempted assassination was intended as a message to other Russian "traitors": "and the message is, 'we will find you'," he said.

What happens next?

The Russians have until midnight Tuesday London time (Wednesday morning AEST) to respond to Ms May’s deadline.

But given Russian officials have already hit back, calling Ms May’s remarks “a provocation” and “a circus show in the British parliament”, it is highly unlikely that Russia is going to meet the PM’s deadline.

“The Russians will deny it and they'll say this is Russophobia,” Dr Sussex said. “Then the ball will be in May’s court as to what she wants to do.”

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Will the UK retaliate?

When Alexander Litvinenko, a former Russian KGB agent who had sought political asylum in the UK, was murdered by poisoning with a radioactive material in 2006, the British government’s response was to expel diplomats.

“This was seen as not going far enough,” Dr Sussex said. So this time, the Brits could recall their ambassador to Moscow, or expel the Russian ambassador. “It’s a very strong step,” he said, “because it means you're cutting off diplomatic relations.”

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Britain has already joined sanctions against Russia that were put in place in 2014 in response to Russia’s annexation of the peninsula of Crimea in Eastern Ukraine, but they could go further with sanctions or other targeted financial actions, which experts believe is the most likely response.

“They can seize the assets of Russian businessmen in London, which would send a message given a lot of those have ties to Putin,” Dr Sussex said. “They could pass their own version of what the Americans have called the Magnitsky Act, targeted sanctions that limit the amount of finance that particular individual Russians can raise.”

What about the World Cup?

Then there’s the option of England boycotting the World Cup in Russia in June if the operation is deemed to have been sanctioned by Russian President Vladimir Putin.

UK foreign minister Boris Johnson first raised this notion last week, before officials wound it back, saying he was calling for a boycott of UK officials, not players and coaches.

Meanwhile, the British tabloids have since claimed Prince William would not attend the event.

“My understanding is if the UK will not send its team to the World Cup then it would not be disqualified, but would not be allowed to play in other [FIFA] games,” Dr Muraviev said.

“They may try to discourage the UK football fans to go to Russia… financially that may hurt the Russians. Or the UK may actually put pressure on FIFA to strip Russia of hosting the World Cup.”

The other countries in the UK - Scotland, Wales and Northern Ireland - did not qualify for the tournament.

It’s difficult to see how English voters would appreciate their government ruining the World Cup for them, and fellow European nations would be unlikely to follow in the UK’s footsteps if it pulled the team out.

“It’s not likely but they can cause a stir,” Dr Muraviev said. “If the British officials may not attend the World Cup there may be suspension of high level diplomatic discussions at prime minister, president, ministerial levels.”

Where are the Americans on this?

The Trump administration’s reluctance to heavily criticise Russia has been the source of much head-scratching and conspiracy theories. So far, the White House has not publicly backed the British in saying they believe the Russians are responsible.

Dr Sussex said US support would be crucial, particularly if the British wanted to get the NATO countries to consider the incident as “a hostile act on the soil of a NATO member country,” and coordinate a NATO-wide response such as more sanctions.

“That’s unlikely because the White House hasn’t followed the British government in saying this was Russia," he said.

“American support will be pretty crucial. This is a big test of whether Trump is prepared to criticise Russia at all. The big elephant in the room is just how much compromising intelligence the Russians have on Trump’s [personal behaviour or business dealings],” Dr Sussex said.