The ultimatum came as Russian officials imposed strict limits on how Scotland Yard detectives will be allowed to operate as they began their investigation in Moscow.

The strict conditions threatened to deepen the diplomatic rift between Moscow and London caused by the death last month by radioactive polonium-210 poisoning of Litvinenko.

John Reid, the Home Secretary, pledged this week that no diplomatic obstacles would stand in the way of Scotland Yard’s investigation. But yesterday Yuri Chaika, Russia’s Prosecutor-General, told the nine British counter-terrorism detectives that they would not be allowed to question senior officers in the FSB, Russia’s secret service.

Whitehall officials are convinced FSB agents orchestrated the poison plot, but Mr Chaika said: “The issue of the FSB authorities is not on the agenda.”

Andrei Lugovoy, the key figure of interest to the police, who was among the last people to see Litvinenko on the day he fell ill, was suddenly admitted to hospital in Moscow yesterday. He claimed that he was too ill with radiation poisoning to speak, but later from his hospital bed said that he had nothing to hide and was ready to meet the detectives.

Even when doctors decide that he is well enough to talk to investigators, the Prosecutor-General says that his men, and not Scotland Yard, will question Mr Lugovoy. In addition, British detectives will have to seek FSB approval to conduct any interviews in Moscow.

Mr Chaika said that during the interviews the British detectives “may participate with our consent, and we might also withhold our consent”.

Any trial of a Russian suspect would have to be in Moscow, he added.

Russian officials also said that the British team would not be able to interview Mikhail Trepashkin, a former FSB agent who is serving a four-year sentence for disclosing state secrets. Mr Trepashkin claims to have vital information about the plot to kill Litvinenko.

At a press conference yesterday Mr Chaika again promised his full co-operation with the British inquiry, but gave little tangible sign that he will make it easy for Scotland Yard. He denied that the radioactive substance used to poison Litvinenko could have come from Russia, and emphasised that Britain would have to provide evidence to that effect before he would open a formal investigation.

Alexander Sidorov, a spokesman for the Russian prison service, said: “Trepashkin is serving a sentence for treason, therefore we cannot allow him to contact foreign security services.”

Prison officials have moved swiftly to punish Mr Trepashkin for “violating regulations”. A district court is to hear an application today to transfer Mr Trepashkin to a tougher, more secure prison, despite concerns from his lawyer about his deteriorating health.

Meanwhile, in Moscow yesterday a search was carried out at the British Embassy for traces of polonium-210 in the room visited by Andrei Lugovoy when he applied for a visa to visit Britain. Experts said they did not expect to find evidence of the radioactive substance.

In England an HPA spokeswoman confirmed that minute quantities of radiation had been found at the Emirates Stadium in North London at “barely detectable levels”. She reiterated previous advice that there was no public health concern, adding that the levels picked up were lower than natural background activity.

In a clear sign of growing diplomatic tensions, the Prosecutor-General appeared to link the Litvinenko investigation to the demands by the Kremlin for Britain to hand over Boris Berezovsky, the exiled oligarch, who is one of President Putin’s fiercest critics.

British courts have thrice rejected Russian requests for the extradition of the billionaire businessman, but Mr Chaika said that he expected a fresh application “in the near term” for Mr Berezovsky and for Akhmed Zakayev, the Chechen separatist leader.

The two men were close friends with Litvinenko.

Last night British diplomats gave a restrained response to Russia’s ultimatum but ruled out any idea of “a swap”.

Last night Litvinenko’s father said his son would be buried on Friday in a sealed coffin in a Muslim ceremony in or near London. Valter Litvinenko said that the family is negotiating with police and the Health Protection Agency on the location.

• Police in Naples last night seized documents and computers from the home of Mario Scaramella, the self-styled Italian defence consultant who was with Litvinenko when he was poisoned, after prosecutors accused him of “illegally dumping waste”.

  Mr Scaramella claims he has evidence that leading Italian left-wing politicians are agents of Moscow. However he is increasingly seen as a figure of diminishing credibility. His claims to be an academic have so far failed to stand up, since none of the universities with which he says he is associated — from Naples to New York — have endorsed him.

  Britain wants to interview

  Andrei Lugovoy Former KGB officer. Worked for a TV station in Moscow run by Boris Berezovsky. Briefly jailed, on release set up business offering bodyguards for wealthy Russians.

  Mikhail Trepashkin Former FSB officer. Investigated 1999 bombings of Moscow apartments, which President Putin blamed on Chechen separatists. Mr Trepashkin claimed FSB was behind the explosions.

  Russia wants to extradite

  Boris Berezovsky Russia’s first billionaire. Mr Berezovsky, 61, fell out with Mr Putin and sought asylum in Britain. Employed Litvinenko and other dissidents. Wanted by Kremlin for alleged corruption

  Akhmed Zakayev Foreign Minister of the Chechen government in exile, he is accused by Russia of terrorist attacks. Mr Zakayev, 50, lived next door to Litvinenko and saw him hours before he fell ill

  Source: Times database

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