The man was spotted on a flight from Hamburg sitting beside Dimtri Kovtun, another Russian whom German police are investigating for trafficking the radioactive material used to poison the former KGB spy.

Officers have studied CCTV footage from airports at Hamburg and London and are understood to believe that the two men were travelling together. However, the mystery figure disappeared after leaving Heathrow with Mr Kovtun. The name he used on the flight and the passport presented to immigration officials does not show up on any hotel register in the capital. It is believed that he met up again with Mr Kovtun in London on November 1, the day Litvinenko fell ill.

Mr Kovtun was one of the last people to see Litvinenko before he collapsed. Scotland Yard will not say if it regards Mr Kovtun as a victim, a witness or a suspect.

German authorities say that traces of polonium-210 were found at a number of locations visited by Mr Kovtun while he was in Hamburg at the end of October. Russian authorities refused German requests to carry out check for polonium on the Aeroflot flight that Mr Kovtun took to Hamburg.

Martin Koehnke, Hamburg’s chief prosecutor, said: “We assume that Mr Kovtun arrived on October 28 on a flight from Moscow and that he was already contaminated with polonium-210.”

He said it appeared that from the moment Mr Kovtun landed at Hamburg airport he started spreading the radioative substance, including to the car sent to collect him. German police are puzzled why no polonium-210 was found on the Germanwings flight that Mr Kovtun and the mystery Russian travelled on to London.

Mr Kovtun remains in a Moscow clinic where doctors say that they are still testing him for radiation poisoning.

Russian police say that they believe Mr Kovtun was also a target for the assassin, and the businessman denies vehemently any role in the poison plot. The British team is reportedly still seeking more information from Mr Kovtun and Andrei Lugovoy, his business partner. Polonium-210 was discovered on two British flights on which Mr Lugovoy travelled to London in October. On October 25 he took BA 875 and stayed at the Sheraton Park Lane. Radiation was found at both locations. It was also found on BA 872, which Mr Lugovoy took on October 31, and at the Millenium Hotel, in Grosvenor Square, where he and Mr Kovtun stayed and where they entertained Litvinenko.

Experts also isolated traces at a third hotel, the Parkes in Knightsbridge, where both men stayed during another trip to London from October 16 to 18 when they flew on Transaero, the Russian carrier.

Mr Lugovoy, who also remains in the Moscow clinic, denies any role in the plot and claims that he is being framed.

Russian authorities say none of the poison was found on the Boeing 737s used by Transaero.

The Russian Prosecutor-General is trying to shift the focus away from Moscow as his officers prepare to travel to London in the new year to interview a number of Russian exiles. They will include Boris Berezovsky, the oligarch critical of Mr Putin, and Akhmed Zakayev, the Chechen separatist envoy.

Friends of Litvinenko fear that the Kremlin will use the investigation to try to settle scores. This week the Prosecutor General announced he wants to question a number of figures from Yukos, the oil giant, whose assets were seized. He named Leonid Nevzlin, a former executive.

Mr Nevzlin fled to Israel in 2003 but flew to New Jersey at the weekend for a holiday in America. Russian officials yesterday asked the US to arrest him. A spokesman for Mr Nevzlin denied any role in the plot.

Mr Nevzlin told The Times how Litvinenko had travelled to Tel Aviv to hand over a file on how Russia’s Federal Security Service planned to claw back millions from wealthy Russians now living in the West.

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