Spy poisoning: Who stands to gain?

"Attacking the family, and someone involved in a swap, is a first," Bruce Jones, a Russia expert with British defence publication Jane's Defence Weekly, told AFP.

Jones said there could be multiple "dividends" for Russia from such an attack as it could be "a warning to anybody who might consider being a traitor".

Skripal, a former colonel in Russian military intelligence, was jailed in his country for betraying agents to Britain's MI6 secret service.

He moved to Britain in a 2010 spy swap and is now hospitalised in critical condition along with his daughter Yulia after they collapsed in the English city of Salisbury.

For Kremlin opponents, the who and the why are obvious.

"Poisoning is the method of choice for the FSB," Russia's security service, said Yuri Felshtinksy, a friend of Alexander Litvinenko, the former FSB agent who was poisoned with a radioactive agent in London in 2006.

A British inquiry into Litvinenko's killing by tea laced with polonium concluded that it was "probably" approved by Russian President Vladimir Putin -- himself a former KGB officer.

Felshtinsky said the Salisbury incident should be viewed in the context of Russia's presidential election on March 18, in which Putin is running virtually unchallenged.

"This has all the hallmarks of a Putin assassination. He is warning anyone in the FSB never to defect, as they'll be hunted down and killed," Felshtinsky said in a statement published in the British media.

Bill Browder, a former investor in Russia who fell out with the authorities and is now a leading Putin critic, said: "Putin does this for demonstration effect."

Browder has led a campaign in memory of his former employee Sergei Magnitsky, who went public with details of massive fraud by Russian officials before dying in detention after spending 11 months in prison in 2009.

Putin "needs to keep everybody absolutely terrified of him," Browder said.

"He doesn't have to kill everybody, he just has to kill a few people and make it so clear that terrible things will come to you if you cross Putin."

Britain 'vulnerable'

Britain's Foreign Secretary Boris Johnson warned on Tuesday the government would respond "robustly" if the poisoning turned out to be the work of a foreign state.

That may have been just the kind of reaction Moscow wanted.

"If there is criticism or sanctions against Russia, that can be leveraged and manipulated by the Kremlin," said Jones.

"If they can provoke a reaction in the UK, it can be used and capitalised upon to prove that Russia is a tragic victim of fate."

Asked why Britain should be chosen as the scene for such a killing, Jones said the country was in "a vulnerable position" because of doubts about Prime Minister Theresa May's leadership and the delicate Brexit negotiations.

Browder said the lack of a strong British reaction to Litvinenko's killing was also a factor.

"In 2016, when it was determined that this was an organised hit from Russia and it was determined by a High Court judge, the government did absolutely nothing," he said.

"That inaction invites Putin to kill people in this country."

Punishing traitors

In Russia too, experts such as Pavel Felgenhauer, an analyst for the Novaya Gazeta newspaper, see Moscow's hand in the poisoning.

"I have no doubt that this was carried out on the orders of Moscow since there are no other parties who could have an interest," he told AFP.

"It is in the traditions of the FSB. They have always thought and they still think that you have to punish traitors to keep discipline in the security services."

But others, like former Soviet spy and writer Mikhail Lyubimov, have dismissed this theory.

"It is just a comedy," he said.

"Who is Skripal? Who cares about him? He was already swapped, meaning he was amnestied. If we wanted to kill him, we would have killed him here, but we freed him".

Alexander Golts, a Russian military analyst, told AFP: "We should not forget that people like Skripal have an adventurous character and nobody knows what kind of adventure he could have got himself into in Britain."