Scotland Yard ran into a wall of officialdom in Russia today in its search for answers to the death of the former spy Alexander Litvinenko.

Despite a formal pledge of cooperation with the team of detectives who have flown into Moscow, Russian authorities threw up a succession of obstacles to the inquiry.

Yuri Chaika, the Prosecutor General, ruled out any interviews with leaders of the FSB, the secret service agency suspected in Whitehall of masterminding the murder of Mr Litvinenko with the radioactive poison polonium-210.

Mr Chaika also insisted that Russian investigators would conduct all interviews with potential suspects and witnesses, reducing the British detectives to little more than observers.

"We are doing the interrogating and they [Scotland Yard] are present at the interrogation, rather than they questioning people in our presence," Mr Chaika said.

He made clear that, during the interviews, the British detectives "may participate with our consent and we might also withhold our consent". Officials would first clarify the legal status of anyone wanted for questioning by Scotland Yard before interviews took place.

Asked whether detectives would be able to question senior officers in the Russian security service, Mr Chaika replied: "The issue of the FSB authorities is not on the agenda."

He announced for the first time that Russian prosecutors plan to interview Andrei Lugovoy, the former FSB major who was among the last people to meet Mr Litvinenko in London before he fell ill.

Mr Lugovoy is top of Scotland Yard’s list of interviewees along with his business partners Dmitri Kovtun and Vyacheslav Sokolenko. All three men met Mr Litvinenko at the Millennium Hotel in London on November 1, but have denied any involvement in poisoning him.

Russian authorities had shown no interest in Mr Lugovoy before the British detectives arrived in Moscow last night. Mr Chaika said there was no question of Britain being allowed to extradite any Russian suspects.

"If they want to arrest them it would be impossible, they are citizens of Russia and the Russian constitution makes that impossible," he said.

Mr Lugovoy checked into a Moscow hospital yesterday despite declaring last week that doctors had found no trace of contamination with polonium-210. Mr Chaika said that Russian officials would help the British team to gain access to him "if the doctors allow it".

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