The boss of the former Russian spy Alexander Litvinenko says he was a traitor who would have been sentenced to death in Soviet times.
Alexander Gusak called him a "direct traitor" for betraying other Russian agents to British intelligence.
Mr Litvinenko, 43, who was a vehement critic of Russian president Vladimir Putin, died on 23 November after being poisoned by radioactive polonium-210.
His friends claim the Kremlin ordered his assassination - Moscow denies this.
Mr Gusak, a former head of the FSB, the successor to the KGB, has now retired and works as a lawyer. In an interview for BBC's Newsnight, he was asked if the spy deserved to die.
He said: "I consider him a direct traitor because he betrayed what is most sacred for any operative - his operational sources.
"For that - and I speak as a lawyer - 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. But that's in accordance with the law."
He said under the previous regime, Mr Litvinenko would have been executed.
"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 the USSR he would have been sentenced to death."
Mr Gusak said one of the agents who believed he had been exposed by Mr Litvinenko offered to assassinate the former spy.
He said: "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?'"
The ex-spymaster also confirmed claims made by Mr Litvinenko of a plan to kill the Russian oligarch Boris Berezovsky in 1997. The plot was later exposed by Mr Litvinenko at a London press conference.
Before his death Mr Litvinenko accused the Russian president Vladimir Putin of ordering his murder.
Police sources have told the BBC that the "most likely poisoner" was Andrei Lugovoi, who met Mr Litvinenko in London on the day he fell ill.
Mr Lugovoi accused the British media of "lies" and said he should be regarded as a witness and not a suspect. Scotland Yard has handed a file on its investigation to the Crown Prosecution Service.