Mikhail Bochkarev was arrested on September 21 at Oslo airport after attending a seminar in Norway's parliament where authorities accuse him of collecting data on the building and its network.

"The investigation has not been able to back up its suspicions," the judge said in the ruling. "It would be natural for them to have found significant elements, if they existed, to substantiate these accusations."

The PST intelligence agency immediately filed an appeal to force Bochkarev to stay in jail while an appeals court examines the case further.

If the release order is upheld there would be no obstacle to him leaving Norway, his lawyer Hege Aakre, told AFP.

The case has riled Russia's government, which has described the charges as "false" and "absurd". Russia's government had summoned the Norwegian ambassador to Moscow in protest.

Norwegian officials regularly accuse Russia of attempted hacking and espionage.

In April, a Norwegian man, Frode Berg, was arrested in Russia also on suspicion of spying.

Berg admitted to having helped the Norwegian intelligence services by acting as a courier several times. Earlier this month, his detention was extended by two months pending trial.