Alexander Litvinenko was a traitor who would have deserved execution in Soviet times, his former chief in Russia’s security service said last night.

Alexander Gusak accused Litvinenko of helping British secret services unmask Russian spies after he fled to London from Moscow. He claimed that furious agents considered assassinating him in revenge.

“I consider him a direct traitor because he betrayed what is most sacred for any operative — his operational sources. His sources came to me and they complained that your [British] secret service officers had found them, and asked what to do,” Mr Gusak said.

Mr Gusak was Litvinenko’s former commander in the Organised Crime Division of the FSB, the successor to the Soviet KGB. He left the service in 1998, the same year that Litvinenko caused a sensation in Moscow by exposing an FSB plan to assassinate the billionaire oligarch Boris Berezovsky.

Mr Gusak, who now works as a lawyer, admitted that there had indeed been talk of a plot to kill Mr Berezovsky. But he insisted that Litvinenko was guilty of betraying his country.

“What Litvinenko did comes under Article 275 of the criminal code. It’s called treason. And there are sanctions; prescribed punishments. Up to 20 years in prison.

“I was brought up on Soviet law. That provides for the death penalty for treason — Article 64. I think if in Soviet times he had come back to USSR he would have been sentenced to death.”

Mr Gusak, who once headed a secret unit described by Litvinenko as a “death squad”, disclosed that he had been approached by FSB agents who believed their names had been passed to the British. “I’ll tell you honestly, I didn’t advise any of them to go and kill Litvinenko, though one of them did say: ‘Listen, he’s done you so much wrong — shall I bring you his head?’,” Mr Gusak told BBC TV’s Newsnight.

Mr Gusak’s decision to come forward with allegations against his former subordinate appears at odds with claims he made last November that Chechen separatists could have killed Litvinenko.

Mr Gusak said then that the murder could have been “blood vengeance” for the death of a captured Chechen militant in 1996. He and Litvinenko had been engaged in an operation against Chechen fighters in Dagestan, southern Russia.

“That evening, when I wanted to interrogate the Chechen, they told me that Litvinenko supposedly tortured him to death,” Mr Gusak told the daily Kommersant paper.

Mr Gusak confirmed Litvinenko’s allegation that a superior officer in their secret unit had ordered them to kill Mr Berezovsky in 1997, but he said that he had not taken the order seriously. He added, however: “If the director of the FSB, Nikolai [Kovalyov], had personally given me the order, I would have carried it out. ”

Mr Kovalyov was replaced as FSB director in 1998 by Vladimir Putin, who was in charge when Litvinenko exposed the plot at his press conference with four other FSB officers, who were all wearing masks to hide their identities.

President Putin has repeatedly dismissed Litvinenko’s allegations of this plot. He stated only last week that Litvinenko had been dismissed from the FSB for abusing detainees and stealing explosives.

Mr Kovalyov is now a senior deputy in the Duma, the lower house of Russia’s Parliament, and former deputy chairman of its security committee. He has already suggested that Mr Berezovsky was behind the murder of Litvinenko as part of an elaborate plot to discredit Mr Putin.

Litvinenko, 43, died in November after ingesting a massive dose of radioactive polonium210. He accused Mr Putin on his deathbed of ordering his execution, a claim strongly denied by the Kremlin.

Scotland Yard has sent a file to the Crown Prosecution Service, which is understood to accuse Andrei Lugovoy, a former KGB officer, and his business partner Dmitri Kovtun of involvement in a conspiracy to kill Litvinenko. Both men met him in London on November 1, the day he fell ill.

Relations between London and Moscow came under further strain last night when Russia complained that Britain was obstructing its attempt to send prosecutors to London to interview people it suspects of involvement in the case, including Mr Berezovsky.

Mikhail Kamynin, a Foreign Ministry spokesman, said that a request for assistance sent on January 8 had stll not been answered. Scotland Yard denied any delay.

Who killed Litvinenko?

The Kremlin favoured theory of anti Putin exiles.

Boris Berezovsky favoured theory of the Kremlin and FSB, the Russian security services.

Chechens advanced by Alexander Gusak, former FSB chief.

Executives of Yukos, the former oil giant advanced by Russia’s prosecutor-general.

FSB veterans advanced by Kremlin critics.

Andrei Lugovoy and Dmitri Kovtun Scotland Yard want to speak further to the former KGB guard and his business partner

Times Recommends

• Michael Jackson: Not the final chapter
• Indian Army spends10m rupees on golf buggies
• Brüno toned down for 15 rating