The radar blip on the flight path of Prime Minister Manmohan Singh's [Images] special plane, which created a mid-air scare, was caused by an Indian Air Force aircraft and the incident cannot be considered a serious security lapse, Civil Aviation Minister Praful Patel said on Wednesday.

"It was not a serious security lapse, and the flight which was spotted on the path of Prime Minister's plane was of the Indian Air Force. The Air Force plane (Dornier) was at a reasonable distance from the PM's plane," he said.

"Primary investigation has revealed that there was some mix-up in the signals," Patel, who was in Ahmedabad to inaugurate the new domestic arrival building at the Sardar Vallabhbhai Patel International Airport, said.

"But, to be sure, we are waiting for the detailed investigation report," he added.

The special aircraft carrying Prime MinisterManmohan Singh had a mid-air scare on Tuesday evening, while it was preparing to land at the Delhi airport, after the Air Traffic Control detected the blip on its flight path on the radar screen.

The Boeing 737 landed safely, though delayed by 16 minutes, but not before the IGI airport was put on high alert and security officials went into a tizzy.