British police investigating the poisoning of the former Russian spy Alexander Litvinenko have arrived in Moscow today to speak to witnesses and collect any evidence relating to his death from their Russian counterparts.

Earlier today, the Russian Foreign Ministry confirmed that visas had been issued to the nine Scotland Yard investigators.

The prosecutor-general's office also offered "to provide all necessary help to British colleagues within the framework of international agreements and the law of the Russian Federation".

British authorities said earlier that the team of detectives would interview several people while in Moscow, including Andrei Lugovoi, another former intelligence agent who met Litvinenko on November 1 - the day he is believed to have fallen ill.

Lawyers acting on behalf of Mikhail Trepashkin, who worked for the FSB, the KGB’s successor, until 1997, said he has key evidence in the case and appealed to the British officers to collect his testimony as soon as possible, saying his life is in danger.

It was reported last week that Trepashkin, who is currently serving a four-year sentence for revealing state secrets, wrote a letter to Litvinenko warning him about a secret squad set up to kill him and other Kremlin opponents.

"Trepashkin said he had information that could shed light on the killing [of Litvinenko]", his lawyer Yelena Liptser told The Associated Press. "If the authorities don’t allow him to do that, that would mean they are trying to hide something."

In a statement dictated on his deathbed, Litvinenko, a former agent for the KGB and the FSB, accused President Putin of involvement in his death. "You may succeed in silencing one man but the howl of protest from around the world will reverberate, Mr Putin, in your ears for the rest of your life," he said.

The spy's father told Kommersant, an independent business daily, that he was also sure that Mr Putin was involved in the death, dismissing suggestions that rogue former agents may have been responsible.

"No kind of veteran’s organisation would dare to kill a former secret service member. There was an order right from the top to kill my son," Mr Litvinenko said. "I am in no doubt that this was done by members of the Russian secret services, with the permission of Vladimir Putin."

Russian authorities have denied any involvement in Litvinenko’s death. Sergey Lavrov, Russia’s Foreign Minister, warned today that continued suggestions of Russian official involvement in Litvinenko’s death could damage relations with Britain.

Mr Lavrov said he had spoken with Margaret Beckett, the Foreign Secretary, "about the necessity to avoid any kind of politicization of this matter, this tragedy," the RIA-Novosti news agency reported.

"If the British have questions, then they should be sent via the law enforcement agencies between which there are contacts," he said.

Meanwhile, one of Litvinenko's former KGB colleagues, Yuri Shvets, said that he had given Scotland Yard information on the death, including the name of the man responsible for the murder.

"The truth is, we have an act of international terrorism on our hands. I happen to believe I know who is behind the death of my friend Sasha and the reason for his murder," Mr Shvets told the Associated Press.

Mr Shvets, who had known Litvinenko since 2002 and last spoke to him on the day that he died, was questioned by British detectives and an FBI agent in Washington last week. He declined to confirm the name of the person he had told police was behind Litvinenko’s death in case it disrupted the investigation.

"I want this inquiry to get to the bottom of it. Otherwise they will be killing people all over the world - in London, in Washington and in other places," Mr Shvets said. "I want to give the police the time and space to crack this case, to allow them to find those behind this assassination, the last thing I want to do is give a warning to those who are responsible."

Litvinenko, 43, died three weeks after ingesting a toxic radioactive isotope, polonium-210, which made his hair fall out and ravaged his organs. Results of the post-mortem examination on his body are expected later this week and might help pinpoint the origin of the radioactive substance.

He is believed to have received the poison at a sushi restaurant in Piccadilly, London. An Italian contact who had lunch with Litvinenko at the restaurant, Mario Scaramella, was also contaminated with polonium, although he has shown no symptoms of radiation poisoning.

In an interview with Italy’s RAI television, Mr Scaramella said that doctors had told him that his body contained five times the dose of polonium-210 that would normally be considered lethal. "So my mood isn’t the best," he told the channel.

Litvinenko's father, Walter, said in an interview published today that his son - who was born an Orthodox Christian but had close links to Islamist rebels in Chechnya - had requested to be buried according to Muslim tradition after converting to Islam on his deathbed.

"He said ’I want to be buried according to Muslim tradition’," Mr Litvinenko told Moscow's Kommersant daily.

"I said, ’Well son, as you wish. We already have one Muslim in our family - my daughter is married to a Muslim. The important thing is to believe in the Almighty. God is one.’"

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