Additional Chief Metropolitan Magistrate Manish Yaduvanshi extended Zubair's police custody by five days.
Zubair, a resident of Kakrala village in Uttar Pradesh, was earlier remanded in two-day police custody on July 18 following his arrest by the crime branch of Delhi Police on July 17.
The police told the court that on July 17, Zubair was apprehended near the parking gate of Kalindi Kunj Park here.
When he was searched, confidential documents and photographs pertaining to Indian army were recovered.
During his interrogation, Zubair had disclosed that he was working for a Pakistan intelligence agency and was in contact with some officials of Pakistan High Commission here and that the recovered documents were to be handed over to that agent of the Commission, police told the court.
The police said Zubair has to be interrogated at length to unearth the entire conspiracy and also to identify the persons who had supplied him the secret documents.