The ex-security agent said he first felt ill after meeting Lugovoi and business partner Dmitry Kovtun at London's Millennium Hotel.

NPR: Kremlin Issues Warning After Expulsions

After his death, suspicion fell on Mr Lugovoi and Mr Kovtun, the two Russians he had met for tea at the Millennium Hotel.

BBC: Who was Alexander Litvinenko?

Traces were also found on two planes at Heathrow airport, at the British embassy in Moscow and at a flat in Hamburg, Germany, linked to Mr Kovtun.

BBC: Who was Alexander Litvinenko?

Mr Lugovoi is represented as an interested person but Dimitri Kovtun, another Russian former agent who was present at a meeting with Mr Litvinenko on 1 November 2006, is not.

BBC: Litvinenko probe to consider alleged Russian link

On Wednesday Russia's Investigative Committee said it had also conducted its own probe, and had ascertained that Mr. Lugovoi and a colleague, Dmitry Kovtun, were also exposed to polonium in the restaurant.

WSJ: Russia Declares Litvinenko Murder Suspect a Victim