Boris Berezovsky, the multimillionaire living in exile in London and an outspoken critic of President Putin, is prepared to meet Russian investigators being sent to London if the Moscow authorities stop obstructing Scotland Yard’s hunt for the assassin of Alexander Litvinenko.

Mr Berezovsky and Akhmed Zakayev, the Chechen dissident who lived in the same North London street as Litvinenko, are among the friends, family and associates of the murdered former KGB agent who are ready to help British detectives make progress with their investigation in Moscow.

Alex Goldfarb, a friend of Litvinenko, said of the exiles sheltering in Britain: “None of them trusts a Russian investigation.

“They will only talk to the Russians in London if the British request that they do it or to help the British get access to witnesses.”

Mr Goldfarb said that Litvinenko’s widow, Marina, and other exiles also want assurances for their own safety from the British authorities.

After a frustrating week in Moscow, Scotland Yard’s counter-terror detectives finally met the key figure in their inquiry for the first time yesterday.

Andrei Lugovoy spent three hours answering questions from Russian prosecutors at a Moscow clinic watched by the British team, who were not allowed to interrogate the former KGB bodyguard turned self-made millionaire.

Mr Lugovoy has denied playing any part in the fatal poisoning of Litvinenko.

After his meeting with the detectives he emphasised that he was not considered a suspect. “I talked only as a witness. I was not accused of anything,” he added.

“I gave full answers to all the questions asked by the investigators. I am ready to meet with representatives of the Russian prosecutor’s office and Scotland Yard detectives again.”

Police still want to know how traces of deadly polonium-210 were found in three London hotels that he used, British Airways flights on which he travelled and offices that he visited.

Along with his business partner, Dmitri Kovtun, he met Litvinenko on the day that the former spy fell ill. Mr Kovtun, a former officer of the Federal Security Service (FSB) was last night warned that he was at the centre of a criminal investigation for allegedly carrying polonium-210 around Europe.

German police say that traces of the radioactive isotope have been found at an apartment in Hamburg, owned by Mr Kovtun’s former wife, where the businessman stayed before flying on to London.

Last night prosecutors in Hamburg described Mr Kovtun as “more a perpetrator than a victim”. His former wife, her two children and her partner have all been exposed to polonium-210.

Mr Kovtun denies playing any part in the poison plot The Russian authorities claim that both Mr Kovtun and Mr Lugovoy were targets for the assassin and say that they are investigating their attempted murders.

It is understood that Russian investigators could travel to Britain as early as this week. Mr Berezovsky, who has been granted asylum, is the principal figure that the Moscow officials want to meet in London. The Kremlin has failed to get the British authorities to send him back to Moscow to stand trial on fraud charges.

A Kremlin official visiting London said yesterday that they believe a joint Anglo-Russian inquiry would be the best way to solve the murder.

Dmitri Peskov, a spokesman for President Putin, told The Times: “It is important that, a couple of days ago, the Russian Prosecutor-General’s Office announced their own case. Now we have two mirror cases from the British and Russian side. It will make cooperation and interaction more comfortable. You cannot imagine secret service people coming independently into another country and doing whatever they wanted.

“The same thing will happen here when the Russians arrive. They will not be walking around London questioning whoever they want. They will do it with their British counterparts.”