An air traffic controller who mistook a genuine request for an emergency landing for a "BS" prank is now the subject of an investigation.

The flight crew on a United Express plane made the request to air traffic control at Denver International Airport last week after they found smoke in the cabin, the Associated Press reports.

But a controller dismissed the call, thinking it was a prank.

Air traffic control only realised the plea was genuine when the aircraft landed and evacuated the passengers on the runway.

The flight crew then made another request for assistance from firefighters to extinguish a fire in the instrument panel.

Colorado television channel KUSA-TV obtained the recorded communication between the cockpit and the tower.

A pilot is heard saying: “Emergency, smoke in the cockpit, roll trucks please,” as the plane made its approach.

The controller then asked: "Who was that?" to which the voice replied with the flight number "5912".

The controller apparently misheard the number and responded after 10 seconds with: "United 12, what's your position?"

When there was no reply, the controller is heard saying: "Did you hear that? I know that's BS. I know it is."

The Federal Aviation Administration is investigating the incident.

One of the passengers on board was taken to hospital.