No injuries have been reported in the incident which occurred at around 5.25 AM. Air India subsidiary Alliance Air's flights to Silchar and Shillong were cancelled as the plane was grounded.
While Air India said the incident was a "gross act of negligence", Jet Airways claimed that the coach accidentally collided with the stationary aircraft.
Airlines sources said the loss, which is being ascertained, will run into crores as besides the actual damage, it will have to remain grounded for some time.
Meanwhile, DGCA has received the preliminary investigation report from its regional office, sources said here.
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"Initial report has been submitted and it suggests that the bus driver taking a nap could be one of the possible causes of the accident," DGCA sources said.
According to officials at the Netaji Subhash Chandra Bose international airport here, the plane was parked at bay 32 and was preparing to leave for Silchar when the driver of the Jet Airways bus lost control of the vehicle which hit the aircraft near its wing.
"AAI (Airports Authority of India), Police, CISF (Central Industrial Security Force) and other authorities have taken serious note of this gross act of negligence and are investigating the incident," Air India said in a statement.
In a separate statement, Jet Airways said, "No one was injured in the collision" and it has started investigation into the incident.
"A coach transporting Jet Airways cabin crew at the Kolkata airport accidentally collided with a stationary aircraft of another airline," it added.