Now traces of polonium 210 are found in Russian businessman's flat

Moscow investigators will fly to London and may interview UK-based opponent of Putin

Russia revealed yesterday that it planned to send its own investigators to London in connection with the Litvinenko affair as Germany became the third country to be dragged into the radiation murder mystery.

It was the first time that Moscow has confirmed it will dispatch its own investigators to the UK, although prosecutors remained tight-lipped about precisely when the visit might take place.

Russia has opened its own investigation into Mr Litvinenko's murder, to run alongside the British inquiry. Russian investigators are keen to question two of his friends in Britain. Moscow has long sought the extradition from the UK of the oligarch Boris Berezovsky and the Chechen rebel spokesman, Akhmed Zakayev. It now has an excuse to interview them as they were both close associates of Mr Litvinenko, who had recently become a British citizen.

Russia's determination to undertake its own inquiries in Britain is unlikely to be welcomed. Andrei Nekrasov, a friend of Mr Litvinenko, said yesterday there was concern among émigrés in London that the Kremlin would use the inquiries as a "pretext to harass exiles".

Meanwhile, German police disclosed that traces of radiation had been found at two properties linked to a key witness in the Litvinenko case.

Officials in Hamburg said they had detected low traces of radiation at a flat linked to Dmitri Kovtun, one of the two Russian businessmen who met Mr Litvinenko on 1 November, the day he was allegedly poisoned.

"There are indications that there has been a source of radiation here, but no source of radiation has been found," said Ulrike Sweden, a police spokeswoman.

Radiation detection experts were also checking the plane that Mr Kovtun flew to London in on the day he met Mr Litvinenko, according to a spokesman for the airline's operator, Germanwings.

Further traces of radiation were found at a building in Pinneberg, in the northern German state of Schleswig-Holstein yesterday, which belongs to Mr Kovtun's former mother-in-law.

Mr Litvinenko, a critic of the Kremlin, was killed in London by a lethal dose of polonium 210, a radioactive substance. He died on 23 November and was buried in London on Thursday.

Mr Kovtun travelled to London from Germany on to meet the ex-spy, he said in an interview with the website stern.de before he was taken to hospital.

Hamburg police said neither Mr Kovtun nor his ex-wife nor her mother were suspects in the investigation.

The traces of radiation could be a sign that a source of radiation had been there previously. Interfax news agency reported last Friday that Mr Kovtun's business partner, Mr Lugovoy, had damage to vital organs consistent with exposure to dangerous levels of radiation. British detectives in Moscow have already questioned Mr Kovtun, along with Russian investigators.