Watch: See David Tennant in the first trailer for Litvinenko

David Tennant has transformed into former Russian spy Alexander Litvinenko for a new ITV show.

The Doctor Who star portrays the ex-KGB officer, who was fatally poisoned in London in 2006, for the four-part mini-series, titled Litvinenko.

The show, which can be streamed on ITVX, dramatises the police investigation into his murder and his widow Marina's 10-year search for justice.

But what is true story behind the shocking case? Here's everything you need to know.

Who was Alexander Litvinenko?

Alexander Litvinenko was a Russian intelligence agent.

In 1988, he joined the KGB, the Soviet Union's vast secret police service, and later worked for its successor organisation, the FSB, where he was tasked with tackling organised crime.

Read more: First image released of David Tennant as Alexander Litvinenko

He defected to the UK and claimed political asylum in 2000 after publicly accusing Vladimir Putin's regime of plotting to kill oligarch Boris Berezovsky.

Who poisoned Litvinenko?

In November 2006, Litvinenko met two businessmen, and fellow ex-KGB agents, for tea at the Millennium Hotel in central London.

The pair slipped Polonium-210, a highly toxic radioactive substance, into his tea, and he was raced to the nearby University College Hospital.

Flanked by his wife Marina (played by Margarita Levieva in the drama), the dissident claimed to have been poisoned on the direct orders of Putin.

Two Metropolitan Police officers, Detective Inspector Brent Hyatt and Detective Sergeant Jim Dawson, were sent to his bedside to interview Litvinenko.

During the police interviews, he provided meticulous details about the events leading up to his illness, which he knew would prove fatal.

He died 23 days after being poisoned, aged 43.

In the years that followed, Marina vowed to get justice for her late husband and successfully pushed for an official public inquiry into his murder.

The inquiry, held in 2016, saw the UK government name the killers as Andrei Lugovoy and Dmitry Kovtun, and admit that the poisoning had "probably" been carried out with Putin's approval.

It found that the use of the radioactive substance, which could only have come from a nuclear reactor, was a "strong indicator" of state involvement.

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The European Court of Human Rights also ruled last year that Russia was responsible for killing Litvinenko.

The Kremlin has always denied any involvement in his death and refused to extradite the two suspects. Kovtun died from COVID in Moscow earlier this year, aged 56.

The full series of Litvinenko is on ITVX now.