Scotland Yard has requested permission from Russia once again to send a team of investigators to Moscow to prove the murder of the former KGB colonel Alexander Litvinenko.

Litvinenko died in a London hospital last November, three weeks after being poisoned by the radioactive isotope polonium-210. From his deathbed, he accused President Putin of having a hand in his death - a charge that the Kremlin refutes.

Yuri Chaika, the Russian Prosecutor-General, said today: "Literally yesterday, a new international investigative request from Great Britain arrived containing a request seeking permission for a group of British investigators probing the Litvinenko case to come to Moscow,."

Metropolitan Police detectives last travelled to Russia in December, but Tony Halpin, Moscow correspondent of The Times, said that Mr Chaika made clear that UK investigators would not be allowed to return until Russian police also probing the Litvinenko death had been allowed to visit Britain.

In a commentary for Times Online, Halpin said that the exiled billionaire Boris Berezovsky and Akhmed Zakayev, the Chechen separatist spokesman, who were both friends of Litvinenko, would be top of their list of interviewees.

He said that Mr Chaika had also tied up the investigation into Mr Putin's long campaign against the fallen oil giant Yukos - many of whose former executives live in Britain.

It was revealed last week that Russian police have asked the Home Office for permission to interview more than 100 people. The Home Office has declined comment on the request.

The intricate police manoeuvrings are the latest indication of the diplomatic tensions raised by the Litvinenko case.

Scotland Yard investigators who went to Russia last month were not allowed to question anyone directly, instead sitting in while Russian authorities conducted the interviews. Russia also began its own investigation, seen as a bid to keep control of the case.

"We have established very good, constructive working relations with the British. They came here, we gave them all possible assistance in the course of their investigation on the territory of the Russian Federation," said Mr Chaika.

Litvinenko fell ill after meeting with Dmitry Kovtun, a Russian businessman, Andrei Lugovoy, another former KGB agent, and Vyacheslav Sokolenko, head of a private Russian security firm, at a bar at the Millennium Hotel in London. All three men have denied involvement in the former agent's death.

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