Sergei Skripal: The story behind the Russian double agent found poisoned on a bench in an English town

Sergei Skripal: The story behind the Russian double agent found poisoned on a bench in an English town

Sergei Skripal: The story behind the Russian double agent found poisoned on a bench in an English town

Updated 8 March 2018, 7:10 AEDT

How did Russian double agent Sergei Skripal come to be fighting for his life after an apparent poisoning in the UK?

A former Russian double agent is fighting for his life alongside his daughter after apparently being poisoned in an assassination attempt in the UK.

Sergei Skripal's journey took him from the secretive world of Russian intelligence to a quiet English cathedral city via a prisoner swap on the tarmac at Vienna airport.

On the way he spied for Britain's MI6 spy agency, was jailed for high treason in Moscow, and was involved in the deal to free Russian "glamour spy" Anna Chapman.

Who is Sergei Skripal?

Skripal, 66, is a former colonel in Russian military intelligence who was arrested in 2004 in Moscow after it was found he had been working as a double agent for the British.

He was accused of passing on identities of Russia's spies onto the UK's Secret Intelligence Service MI6 in return for $US100,000.

The information exposed a huge network of Russian military spies working across Europe.

He was charged with "high treason in the form of espionage" and sentenced to 13 years in prison.

At the time of his conviction, military court spokesman Yevgeny Komissarov said "his activities caused a significant blow to Russia's external security".

Enter Anna Chapman and The Illegals Program

While he was languishing in a Russian cell, events were underway in the USA which were to see Skripal gain his eventual freedom.

A network of Russian sleeper agents dubbed The Illegals Program by US authorities were posing as ordinary Americans and trying to elicit useful information from policymakers, analysts and business people.

The 10 agents were arrested in 2010 after a decade-long FBI operation.

The most high-profile of the group was red-haired New York socialite Anna Chapman.

A diplomat's daughter, formally known as Anna Kushenko, Chapman once worked at Barclay's bank and gained a British passport after marrying a British citizen.

The phrases "flame-haired", "Bond Girl", "sexy spy" and "femme fatale" were liberally applied by the press.

But Russia wanted Chapman and the rest of the Illegals back.

So, in a prisoner swap reminiscent of the Cold War, Chapman and the rest of the Illegals were swapped for four double agents being held by the Russians — including Skripal.

Then Russian president Dmitry Medvedev pardoned Skripal in exchange for the 10 Russian agents.

The spies were exchanged on the tarmac at Vienna airport.

The returning Russian spies were greeted as heroes in Moscow while Skripal was cast as a traitor.

A new life in England

As part of the trade deal, Skripal was granted asylum in the UK where he had kept a low profile for the past eight years.

He had been living openly in Britain under his own name, and his address in Salisbury appeared in public records.

On Sunday, he was found slumped unconscious on a bench at a Salisbury shopping centre alongside his daughter Yulia, who was reportedly visiting him from Moscow.

The pair are fighting for their lives in hospital, while a member of the emergency services who treated them is also unwell.

UK Foreign Secretary Boris Johnson has warned Russia is a "malign and disruptive force in the world" and has warned foreign governments that "no attempt to take innocent life on UK soil will go either unsanctioned or unpunished".

Why it's a big deal

Relations between Britain and Russia have been strained since the murder of ex-KGB agent Alexander Litvinenko in London in 2006.

The news of Skripal's apparent poisoning invokes a sense of deja vu for the British because how closely his story resembles that of the targeted assassination of Litvinenko.

Litvinenko fell ill in November 2006 after drinking a tea laced with the rare radioactive substance polonium-210 and died three weeks later.

A British inquest concluded that Litvinenko had been murdered by Russian agents with the probable approval of Russian President Vladimir Putin.

In a sign of how seriously the British are taking this, Mr Johnson issued a veiled threat to pull England out of the football World Cup — whish is being held later this year in, you guessed it, Russia.

Mr Johnson also pointed to further sanctions against Russia, at a time when relations are already described as being at their lowest point since the Cold War.

The 'poisoned pint' theory

The investigation has entered its fourth day and Britons have woken up to front pages splashed with new theories on what happened to Skripal.

The Daily Telegraph claims Vladimir Putin always wanted Sergei Skripal killed and had "sworn revenge" on him after he was sent to the UK.

The Sun reports that a "Kremlin assassin" may have spiked Skripal's pint of beer with a "rare and almost untraceable rat poison".

The Daily Mail has reported on a social media post from Yulia Skripal in which she criticised Vladimir Putin and said he should be sent to prison.

And we're all waiting to find out what happens next.