It was the first time that Moscow has confirmed it will definitely dispatch its own investigators to the UK, though prosecutors remained tightlipped about precisely when the visit might take place. Russia has opened its own investigation into Litvinenko's murder that is running alongside the British probe, and is known to be keen to question two of the late man's friends in the UK.

Moscow has long sought to extradite UK-based oligarch Boris Berezovsky and Londonbased Chechen rebel spokesman Akhmed Zakayev and now has an ideal excuse to interview them as they were both close associates of Litvinenko. Russia's determination to undertake its own inquiries in the UK is unlikely to be welcomed though, and Andrei Nekrasov, a friend of the late Litvinenko, said yesterday there was concern among emigres in the British capital that the Kremlin would use the inquiries as a "pretext to harass exiles in London".

Meanwhile, German police disclosed that traces of radiation have 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 Litvinenko on the day he was allegedly fatally poisoned, 1 November. "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.

Another flat linked to Kovtun, in Haselau, west of the port city, also tested positive for contamination.

Kovtun, who is currently in an undisclosed Moscow clinic being treated for "acute radiation sickness", spent time at the German flat before travelling to London to meet Litvinenko.

Traces of polonium-210, a radioactive isotope, have also apparently been detected on a teacup at the Millennium Hotel in Mayfair, London, where the three men met on 1 November.

The other Russian businessman involved in the case, former FSB security service officer Andrei Lugovoi, is regarded as a star witness but he too appears to have absorbed a large dose of radiation. He is apparently being treated by specialists more used to working on victims of the Chernobyl nuclear disaster and nuclear submarine accidents.

Both Russians have strongly denied any involvement in Litvinenko's poisoning, pointing to the fact that they too have been badly contaminated with radiation and are victims in what they say is a plot to frame them. Russian prosecutors have opened a separate investigation into what they have termed as "the attempted murder" of Kovtun in London.

A nine-strong team of British investigators who have been in Moscow since last Monday have been urgently trying to sit in on a questioning session with Lugovoi. But their efforts to do so have been repeatedly frustrated with planned interrogations cancelled three times in as many days.

Indeed the team of experienced anti-terrorism specialists appears to have had a fruitless and faintly humiliating week. When they arrived at Moscow airport on Monday they were immediately photographed by the Russian media and subsequently had their names and pictures splashed across newspapers despite the 'sensitive' nature of their mission.

Dressed almost exclusively in stereotypically black detectives' raincoats and almost all male, they looked startled and embarrassed to have received such a public welcome. In the days that followed, their biographies and even details of their abilities to speak Russian (or not) were published.

The detectives appear to have spent more time meeting their Russian counterparts and being told what they cannot do than carrying out genuine investigative work. After five days the only questioning session they are known to have sat in on is one with Kovtun.