The driver had dropped off his employer at the airport and was going to Gurgaon when he was flagged down by a man wearing a Delhi Police uniform with two star badges on his shoulder.
After the 'policeman', identified later as Bharat Singh, sat in the car, he first asked for the driver's cell phone. Then, he whipped out a knife and threatened to kill him, the police said.
The driver got off the BMW and the robber fled with the vehicle with the mobile still inside.
Singh had joined the Navy as a seaman and retired as a petty officer in 2001.
The police has seized the uniform he had bought from Kingsway Camp, a knife and the robbed cell phone of the complainant.
They are probing whether he had any accomplices, who were going to help him in disposing the robbed car.