It is revealed as part of a Telegraph investigation which 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 worlds most dangerous criminals.

The tape will reignite claims that Litvinenko could have been killed as a result of investigative work he carried out in a series of European countries after leaving Russia.

These claims are likely to be played out in the High Court as the Litvinenko Inquiry, chaired by Sir Robert Owen, a former high court judge, conducts its hearings.

Last year Sir Robert said that he had seen prima facie evidence that the Russia state was involved in the murder.

It is likely that the NSA intelligence formed part of the evidence that Sir Robert was given.

The disclosure of the material is likely to be put pressure on the British governments relationship with the Kremlin and will renew calls for the UK to toughen its stance.

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

Mrs Litvinenko has applied to the NSA to disclose telephone intercepts, and says that all information should be disclosed to Sir Robert.

Litvinenko was poisoned in November 2006 during a meeting at a Mayfair hotel. He died three weeks later. Tests revealed that he ingested a rare isotope, polonium 210, which is hard to detect.

British prosecutors want two men, Andrei Lugovoy and Dimitri Kovtun, both of whom are former KGB bodyguards, to face murder charges over the murder.

Read the original:

Litvinenko inquiry: the proof Russia was involved in dissident's murder