A filling unit for filling a bottle with a food liquid; the unit comprising: a tank adapted to contain a food liquid to be introduced into the bottle; a filling device for filling by level, which includes: a body having an inner cavity which extends vertically through said body; an outer tube which communicates with the inner cavity of the body, protrudes downwards with respect to the body and has a lower terminal section configured to guide the liquid into the bottle to be filled; an inner tube, which is arranged coaxially with the inside of the outer tube, has a lower section protruding outside the outer tube and is configured to be inserted inside said bottle, protrudes upwards relative to the outer tube, so that its upper end, when fitting the filling assembly on the machine, is above the level of the liquid contained in the tank, the outer tube and the inner tube delimiting an annular duct which extends to the lower end of the outer tube and through which the liquid contained in the tank flows downwards to fill the bottle; a centring cone which is arranged coaxially with the outer tube and the inner tube and is configured to provide a seal against the upper end of the neck of the bottle to be filled; a first mobile shutter configured to close the annular duct in a sealed manner, a second shutter configured to close the inner tube in a sealed manner; first operating means configured to control the movement of the first shutter; second operating means configured to control the movement of the second shutter; characterized in that the filling unit comprises third operating means configured to vary the height position of said lower terminal section of the inner tube inside said bottle; the filling unit further comprising a flow meter adapted to measure the volume of fluid passing through said annular duct and configured to emit a delivery signal proportional to the volume delivered; and command and control means configured to emit a command and control signal of said second and third operating means in response to said delivery signal. a tank adapted to contain a food liquid to be introduced into the bottle; a filling device for filling by level, which includes: a body having an inner cavity which extends vertically through said body; an outer tube which communicates with the inner cavity of the body, protrudes downwards with respect to the body and has a lower terminal section configured to guide the liquid into the bottle to be filled; an inner tube, which is arranged coaxially with the inside of the outer tube, has a lower section protruding outside the outer tube and is configured to be inserted inside said bottle, protrudes upwards relative to the outer tube, so that its upper end, when fitting the filling assembly on the machine, is above the level of the liquid contained in the tank, the outer tube and the inner tube delimiting an annular duct which extends to the lower end of the outer tube and through which the liquid contained in the tank flows downwards to fill the bottle; a centring cone which is arranged coaxially with the outer tube and the inner tube and is configured to provide a seal against the upper end of the neck of the bottle to be filled; a first mobile shutter configured to close the annular duct in a sealed manner, a second shutter configured to close the inner tube in a sealed manner; first operating means configured to control the movement of the first shutter; second operating means configured to control the movement of the second shutter; characterized in that the filling unit comprises third operating means configured to vary the height position of said lower terminal section of the inner tube inside said bottle; the filling unit further comprising a flow meter adapted to measure the volume of fluid passing through said annular duct and configured to emit a delivery signal proportional to the volume delivered; and command and control means configured to emit a command and control signal of said second and third operating means in response to said delivery signal.