Feeding unit (UG) for feeding a plastic film in packaging machines, comprising: a unit (1) for unwinding the film from a reel (2); a cutting device (3) downstream of the unwinding unit (1) and intended to form sheets (SF) of predetermined length; a transfer device (4) downstream of the cutting device (3); a positioning device (5) downstream of the transfer device (4) and comprising drive belts (50, 51) able to engage the upper and the lower faces of the film. The belts of the positioning device are controlled by a control unit (UE) connected with optical means (OC) that detect the passage of signs (T) formed on the film (F). The control unit (UE) is programmed to compare a transit time elapsing during the passage of the signs (T) of a same side of two consecutive sheets (SF) with a cycle time and to increase or decrease the speed of said belts (50, 51) if the transit time is less than or greater than the cycle time.