British detectives investigating the murder of Alexander Litvinenko have wound up their operation in Moscow after a fortnight of frustration and obstruction by Russian officials.

As the Scotland Yard team prepared to fly home, the family and friends of the former KGB agent pleaded with the British authorities not to abandon their hunt for the assassins.

Some of the nine strong counter terror team returned over the weekend leaving the rest to carry out a final interview with a Russian businessman, Dmitri Kovtun, who entertained Litvinenko just hours before he fell ill.

Mr Kovtun was interviewed for the third time yesterday at a Moscow clinic where he is reportedly being treated for exposure to polonium-210, the radioactive poison that killed Litvinenko.

German investigators say they have begun a criminal enquiry into Mr Kovtun after traces of polonium-210 were found at several locations he visited in Hamburg.

Last night friends of Litvinenko, who was given British citizenship just weeks before he was killed, urged the Yard detectives to continue their work in Moscow.

Alex Goldfarb, who was at Litvinenko’s bedside during his final days, told The Times: "We hope the British authorities have enough willpower and clout to follow this through to wherever it leads.

"It was clear from day one that the Russian authorities would obstruct this investigation. This only adds to the suspicion they were behind this murder and are covering up."

"We want Tony Blair to live up to his promise that no political or economic obstacle will get in the way of the police enquiry."

The British team have interviewed six people while in Moscow. They include Mr Kovtun’s business partner - Andrei Lugovoy - who was also with Litvnenko on November 1, and another Russian businessman also staying at the Millenium Hotel, Vyacheslav Sokolenko. Both deny any involvement in the plot.

Police also questioned a doctor treating two of the Russians for radioactive contamination as well as Mr Lugovoy’s wife and solicitor.

Police sources say they will review the evidence gathered in Moscow then decide whether to return to Russia.