A court in St Petersburg has suspended the treason trial of a prominent Russian environmental campaigner and ordered prosecutors to conduct further investigations.

The chairman of the judges' panel described prosecution evidence as unclear.

The campaigner, Alexander Nikitin -- a former submarine officer -- was arrested three years ago, accused of providing the Norwegian environmental group, Bellona, with secret information for a report on deteriorating Russian nuclear vessels.

Bellona says the information was publicly available -- some of it from school textbooks.

A defence lawyer for Mr Nikitin was quoted as saying prosecutors may now have to drop the case, because, he said, there were no more facts to investigate.

Prosecutors have seven days to lodge an appeal.