Mr Nikitin was arrested in February 1996 as part of an investigation by Russian security services into the activities of the Norway-based environmental group Bellona in the port of Murmansk.

Mr Niktin alleged that old Russian subs were a pollution hazard

After a four-year legal battle, Mr Nikitin was acquitted in December by a St Petersburg court - a decision which the Supreme Court upheld on appeal on Monday.

Mr Nikitin, 46, was accused of using his officer's identity card to gain access to a military unit in St Petersburg, where he was alleged to have consulted top secret documents.

In a 1996 report for Bellona, Mr Nikitin wrote about 52 abandoned nuclear submarines in a remote shipyard near Russia's border with Norway.

He alleged that the submarines - which hailed from the Cold War era - held spent nuclear fuel that could leak, overheat or explode.

'Prisoner of conscience'

Prosecutors with the Federal Security Service - the successor to the KGB - had demanded that Mr Nikitin be sentenced to 12 years in a labour colony and have his property confiscated.

One of the most controversial criminal cases in Russia since the collapse of the Soviet Union

Amnesty International

The former submarine officer spent the first 10 months of his four-year legal battle in a St Petersburg prison before he was freed and put under house arrest.

Mr Nikitin and Bellona said the information was not secret but came from public records, including school textbooks.

Mr Nikitin was declared a prisoner of conscience in 1996 by Amnesty International, which described the case against him as "one of the most controversial criminal cases in Russia since the collapse of the Soviet Union".