Abstract: Feeding unit for feeding a plastic film in packaging machines, including: a unit for unwinding the film from a reel; a cutting device downstream of the unwinding unit and intended to form sheets of predetermined length; a transfer device downstream of the cutting device; a positioning device downstream of the transfer device and comprising drive belts able to engage the upper and the lower faces of the film. The belts of the positioning device are controlled by a control unit connected with optical elements that detect the passage of signs formed on the film. The control unit is programmed to compare a transit time elapsing during the passage of the signs of a same side of two consecutive sheets with a cycle time and to increase or decrease the speed of said belts if the transit time is less than or greater than the cycle time.