The questioning, according to IANS, began at the probe agency's MTNL building office after Puri arrived there after 11 am, following summons issued to him last week to join the probe.
Ratul Puri is the Chairman of the Board of Directors of Hindustan Powerprojects, one of India's leading integrated power players in the energy sector. He is the son of Nita and Deepak Puri, the CMD of optical storage media firm Moser Baer. Nita is Nath's sister.
The central agency had on Wednesday informed a Delhi court that Puri has been summoned in this high-profile case for interrogation.
The ED had also informed the court that Puri was summoned for confronting him with Sushen Mohan Gupta, an arrested middleman in the case, whose custodial interrogation was Wednesday extended by three more days by the court.
The businessman, however, has denied his role in the deal. "I am cooperating in the investigation and have no connection in the defence deal," Puri told media before joining the probe.
In a statement, his company had said: "He would by fully cooperating with the ED investigation and provide any clarification or information as may be required."
The about Rs 3,600 crore deal to purchase AgustaWestland choppers for the transport of VVIPs was scrapped by India over allegations of corruption and kick backs being paid in the deal. The ED and the CBI are probing the case now and have already filed multiple charge sheets in the case.
Kamal Nath has questioned the timing of the ED summoning his nephew.
Saying he had nothing to do with his nephew's business, Nath, however, asked why the development came ahead of the Lok Sabha elections.
"He (Puri) is independent and has nothing to do with politics, and I have nothing to do with his business. Whatever (allegation) there is, a full investigation should take place. I welcome it...but why is it emerging during elections?" Nath told reporters.