Although he had a position in Russia's embassy there, his real job was to glean classified information about Nato.

Sources say he had been groomed for months by an MI6 agent posing as an art dealer, after expressing a desire to defect over his belief that Russian president Vladimir Putin is determined to foster conflict with the West.

Last night a recently retired MI6 agent revealed Apollo was brought to Britain in early April and debriefed.

He provided Secret Intelligence Service officers with a "wide spectrum of routine and sensitive information" about Moscow's worldwide spy network.