Alexander Litvinenko was poisoned as he was gathering evidence to incriminate Russian President Vladimir Putin and Russian top officials for war crimes in Chechnya, Chechen separatist leader Akhmed Zakayev has said.
In an interview with IBTimes UK, Zakayev said Litvinenko was a leading figure in a Chechen government commission investigating alleged atrocities committed by the Russian military during the two Chechen wars.
"He was very important, absolutely indispensable in this commission because he knew all the generals who took part in military campaigns in the first and second war," Zakayev said.
"He also knew people in the FSB [Russia's intelligence agency] who gave him information on this matter and what else was happening in the northern Caucasus."
Tens of thousands of people, including many civilians, were killed during the two Chechen conflicts of 1994 - 1996 and 1999 - 2000.
Grozny was heavily bombarded in the second war that came as Putin was prime minister and helped him consolidate his power in Moscow.
Thanks to his insider knowledge, Litvinenko was reportedly able to link generals and other officials to specific attacks that resulted in high civilian casualties.
Zakayev, a close friend of Litvinenko said murdered journalist Anna Politkovskaya was also a member of the investigative team set up by Chechen president Aslan Maskhado in 2004.
Their work angered the Kremlin and eventually contributed to a death sentence being passed on the former KGB agent, Zakayev claimed.
Litvinenko was poisoned with highly radioactive polonium-210 at a London hotel In November 2006. Politkovskaya was shot dead in Moscow on Vladimir Putin's birthday a month earlier, while Maskhado was killed by Russian security forces in 2005.
Material gathered by the commission was to be used to file a war crimes case against Putin and Russia's top brass with the International Crime Court (ICC) in The Hague.
A criminal complaint was eventually sent to the ICC in 2014 by the unrecognised Chechen Republic of Ichkeria, of which Zakayev is the Prime Minister-in-exile.
The Moscow-educated former actor was a leading rebel commander during the first Chechen war and is due to be heard as a witness at the ongoing inquiry on Litvinenko's poisoning in the coming weeks.
A friend of actress Vanessa Redgrave, Zakayev first met with Litvinenko in London in the early 2000s after they were both granted asylum by Britain.
Sharing a common hatred for Putin and empathy for the Chechen cause, the two became friends and neighbours.
"He [Litvinenko] got very involved with our struggle," Zakayev said. "He once told me: 'Just like Germans now feel ashamed of what was done to the Jews, so Russians will feel ashamed of what has been done to Chechens. I don't want my son to be ashamed for me and that is why I would like to support the Chechens in their fight'."
On his London deathbed, Litvinenko converted to Islam and expressed the desire to be laid to rest in the same Chechen graveyard where Zakayev will also be buried.
The 55-year-old political dissident argued that his friend's fierce and vocal criticism of the Kremlin eventually led to his poisoning.
"He was seen as a traitor by the FSB not because he betrayed Russia but the cause that they [the FSB] served," he said.
While allegedly working for MI6, the former spy wrote a book claiming the Russian agents were behind the 1999 apartment block bombings in Russia that killed more than 300 people and eventually resulted in Putin - the former FSB chief - sending troops into Chechnya. Russia has always maintained Chechen separatists carried out the attacks.
In January 2015, the inquiry into Litvinenko's death also heard how the murdered spy had penned a series of critical articles including personal attacks against Putin as well as claims that corruption was endemic amid Russian intelligence officers.
In a piece published on a Chechnya website, Litvinenko accused the Russian president of being a paedophile after he kissed a five-year-old boy on the stomach.
Earlier this week, the former spy's widow Marina told the inquiry how her husband's activities resulted in what she described as a Kremlin-led campaign of intimidation against her family. Marina told the inquiry how Litvinenko's house and the neighbouring property of Zakayev were firebombed in 2004.
The separatist leader has also been the target of an alleged assassination plot. In 2012, the MI5 issued a warning that he was high up in a hit list drawn up by the Ramzan Kadyrov, Chechnya's Kremlin-backed current president - a man that rights groups accuse of overseeing murders and torture of opponents, and who has a tiger as a pet.
Zakayev, the last surviving top moderate rebel commander from the first Chechen war said he is well aware of the risks connected to his political activity, here in London, but he sees no point in dropping the fight for an independent Chechnya.
"If you get involved into Putin's crimes you get onto the list of those who might get killed," he said. "This danger exists and has always existed. However it doesn't mean that we all have just to hide and keep silent.
"Fear is not a guarantee of security. The guarantee is to make the regime in Russia change. If there is a different regime we will all be safe."