Washington: American spies secretly intercepted communications between those involved in the murder of Alexander Litvinenko and provided key evidence that he was killed in a Russian-backed “state execution”, The Daily Telegraph can disclose.

The National Security Agency (NSA) obtained electronic communications between key individuals in London and Moscow from the time the former spy was poisoned with radioactive material in central London.

The evidence was passed to the British authorities.

A source familiar with the investigation confirmed the existence of American “intelligence material”. They said it would have been “inadmissible” in court, but that the British authorities were “confident that this was a state execution”.

The disclosure comes ahead of the start on Tuesday of the public inquiry into Litvinenko’s death in 2006, which will see hearings, many of which will be held in secret, carried out over a nine-week period in the High Court. The existence of the American intelligence material offers the first proof that the Russian state was involved in the murder of the dissident, and explains why senior British politicians have been so confident in publicly blaming the Kremlin for the murder.

The Telegraph investigation has also unearthed an audio recording appearing to capture Litvinenko giving a detailed account of his investigations into links between Vladimir Putin and one of the world’s most dangerous criminals.

The tape, published on telegraph.co.uk, will reignite claims that Litvinenko could have been killed as a result of investigative work he carried out in a series of European countries. These claims are likely to be played out as the Litvinenko Inquiry, chaired by Sir Robert Owen, a former high court judge, conducts its hearings.

Last year, Sir Robert said he had seen “prima facie” evidence the Russian state was involved in the murder. It is likely that the NSA intelligence formed part of the evidence that Sir Robert was given.

The start of the inquiry comes after years of campaigning for an official verdict on Litvinenko’s death by his widow, Marina.

The former KGB spy was poisoned in November 2006 during a meeting at a Mayfair hotel. He died three weeks later.

Tests revealed that he had ingested a rare isotope, polonium 210, which is hard to detect.

British prosecutors want two men, Andrey Lugovoy and Dimitry Kovtun, both of whom are former KGB bodyguards, to face murder charges. Lugovoy, who is now a Russian MP, and Kovton have always maintained their innocence, and Moscow has said that they cannot be extradited under Russian law. An international warrant has been issued for their arrest if they ever leave Russia.

Last October, Mrs Litvinenko filed a Freedom of Information request through an intermediary to the NSA asking for “intercepts of telephone communications of Andrey Lugovoy and Dimitry Kovtun from London, UK, in the period October 15 to November 1 2006.”

The application stated the material was “to be used as evidence in the [public] inquiry hearings”.

Paul Blaskowski, a senior NSA official, responded in a letter that the body could not comment on the “existence or non-existence” of the transcripts because such material had “to be kept secret in the interest of national defence or foreign relations”.

He said the spy agency was also empowered “to protect certain information concerning its activities” by withholding it from public disclosure. Joel Brenner, who was inspector general of NSA at the time of Litvinenko’s murder, said that the “cooperation between the UK and US government on signals intelligence is extremely close and probably without parallel”.

The public inquiry was ordered by Theresa May, the Home Secretary, last year. It replaces an ongoing inquest and one of its key purposes will be to examine whether the Russian state was behind the killing.

The government had previously resisted calls for an inquiry, citing the need in part to “protect international relations”.

But Sir Robert, who also acted as coroner at the inquest, said he could not conduct a “fair and fearless” investigation because the government had refused to release certain information.

This is thought to be intelligence material relating to the involvement of Russia in the case, and of Litvinenko’s work as an informant to MI6.

Sir Robert said last July that “sensitive” government evidence over the poisoning would now be heard in closed sessions of the public inquiry.

The possible involvement of the Russian government in the murder would be of “central importance to my investigation”, he added.

According to information that came out during the inquest, Litvinenko had been working for MI6 for several years during his time in London. As part of this, Litvinenko also began assisting the Spanish security services. It is understood that his work in Spain involved investigating organised crime networks. Litvinenko’s work in Spain — as well as in Italy and Georgia — after leaving Russia and the KGB has given rise to competing theories about who might have been behind his death. The disclosure of the former spy’s verbal account of his investigations of Moscow’s links to criminal networks in Italy will raise fresh questions about the risk involved in his work in the country.

— The Daily Telegraph