Captain Grigoriy Pasko was convicted of the lesser charge of military misconduct and sentenced to three years in jail.

But he was set free by the military court under the terms of a state amnesty because he had already been in custody for 19 months.

After his release he told the BBC that his work on ecological issues was only one of several reasons for his arrest by the Russian security services.

He said that the charge of abuse of power on which he had been found guilty was complete nonsense.

The judges noted numerous violations during his arrest and search by police in November 1997.

BBC Moscow Correspondent Andrew Harding: "A humiliating defeat for the Russian security services"

Captain Pasko was detained after filing a report for a Japanese television station which purported to show Russian sailors dumping radioactive waste into the Sea of Japan.

Prosecutors said Captain Pasko had sold vital military secrets, but his lawyers accused the security services of tampering with evidence.

In his final statement to a military court in the far eastern city of Vladivostok Captain Pasko said the charges were fabricated in revenge for his exposure of environmental pollution by the Russian Pacific fleet.

After the ruling he said:"Right now I am going back to prison, but this time only to pack up my things and tell all of my fellow inmates, who were rooting for me very much."

Grigoriy Pasko: supported by Amnesty

The environmental journalist's plight attracted the attention of world human rights organisations, including Amnesty International which declared Captain Pasko a prisoner of conscience.

After a trial of six months, the court handed down a sentence of three years in prison, and then immediately released Captain Pasko under an amnesty bill passed by the Russian Parliament earlier this year.

'Legal fudge'

Under the amnesty, Captain Pasko's release was ordered because he had already spent more than a third of his sentence term in prison.

The BBC Moscow Correspondent, Andrew Harding, says the verdict looks like a rather delicate legal fudge, but Mr Pasko's lawyers have already said they intend to challenge the sentence and clear their client's name.

Details of the charges against him remain sketchy, but prosecutors have said that he sold information about the combat readiness of the navy.

They initially requested a 12-year sentence for treason and espionage, claiming that the officer had sold military secrets obtained while on duty.

The court instead followed the defence's line, saying that several criminal violations had been committed during the investigation into him.

Among others offences, two intelligence service documents presented as evidence had been falsified, the court found.

"The facts of collection and storage of information for transfer to Japanese media have not been substantiated," Pacific Fleet Judge Dmitry Savushkin said as he read the verdict to the case.

'Stalin-like regime'

In his final statement to the military court Captain Pasko reiterated that he was neither a spy nor a traitor and accused federal officers who investigated the case of breaking the law.

"I was just doing my duty as a journalist," he said, arguing that he was the victim of a Stalin-like regime.

His diary, describing the 19 months he spent in Russia's overcrowded prison system, has been published on the Internet.