Russia’s top prosecutor has told Scotland Yard that it must let the Kremlin’s investigators question critics of President Putin living in Britain before British detectives can return to Moscow to finish investigating the murder of Alexander Litvinenko. Yuri Chaika delivered his ultimatum as he revealed that detectives have asked to visit Moscow again to question a number of suspects about the murder of the former spy.

Mr Chaika, the Russian Prosecutor-General, dramatically raised the stakes by suggesting that the Kremlin expects to find the killer among the wealthy Russians given asylum in Britain.

He said: “We don’t rule out that the murder may have been committed by Russian citizens who live abroad.”

He added that he suspected that other Russians may have been fatally poisoned in London. He is asking Scotland Yard to investigate the sudden deaths in Britain of at least three Russians.

The demands are likely to create a further strain on relations with Britain, already damaged after Litvinenko made a statement on his deathbed accusing the Kremlin of ordering his assassination.

While the detectives, who spent a fortnight in Moscow before Christmas, believe that the killer is in Russia, their counterparts are expected to point the finger at Mr Putin’s enemies sheltering here.

The chief prosecutor made it clear that there was no chance that British police would be allowed to return to Moscow before his team have finished their work in Britain.

The Russians have asked to question up to 100 people. A number are reluctant and have asked British police for firm assurances about their safety.

Mr Chaika revealed that the main target for his investigators was Boris Berezovsky, the exiled billionaire who is a critic of the Kremlin and was a friend of Litvinenko.

Pro-government newspapers in Russia have speculated that Mr Berezovsky was behind the poisoning, an allegation that he has denied.

Last night Alex Goldfarb, a close friend of the tycoon, said that Mr Berezovsky believed that the Russian investigation was “a farce” and he was fearful that he could be poisoned while being questioned.

As he has asylum here, he does not have to meet the Russian officers.

“All of this is an exercise in obstruction,” Mr Goldfarb said. “He will speak if the security is provided — if the British guarantee there is no polonium.

“He understands that the Russians are holding the investigation hostage. They are trying to create a situation where the British would not be able get access to the suspects.”

Mr Chaika has raised the case of the death in London nine days ago of Yuri Golubev, a founder of the energy giant Yukos, whose assets were seized by the Kremlin. Police are sure that Mr Golubev, who was 65, died of a heart attack.

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