A careless robotics expert caused a panic at Newark Airport yesterday when he left what looked like C-4 explosive in the trunk of his rental car, authorities said.

“It was a really frightening thing,” said one Budget Rent-A-Car employee.

The absent-minded genius works for iRobot — a company that manufactures robots for bomb disposal — and the suspected explosive was actually just a realistic-looking model used for sales presentations, said a Port Authority spokesman.

The model was found by terrified rental-car workers at about 3 p.m. The expert boarded a flight to Boston and was detained upon landing and later released, a PA official said.

“This is a ‘Big Moron Story,’ ” said a PA official, who added that the expert was supposed to ship the model home but forgot.

The PA plans to bill iRobot for the overtime and bomb squad costs.