An investigation into the sudden plunge of the Serbian president's plane last week shows that it was due to the co-pilot spilling coffee on the instrument panel.

The investigation showed the co-pilot accidentally activated the emergency slot extension when trying to clean the coffee from the panel. That reportedly caused the plane to dive and the brief shutdown of one of the engines.

The plunge, which happened on Friday while the jet was cruising towards Rome over the Adriatic sea, caused panic among the passengers until the captain stabilised the aircraft and returned to Serbian capital Belgrade.

President Tomislav Nikolic was heading to the Vatican for an official visit, which was cancelled.

At the time his press office said the Falcon 50 plane made an emergency return because of an engine failure.

"We lived through a real drama in the air," his adviser Ivan Mrkic said. "The plane started shaking violently. We were being tossed all over in the cabin. We are alive only thanks to the pilot."

Serbia's aviation authorities say the co-pilot has been suspended.

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