Abstract: In a motor vehicle having a fuel tank fitted under the body, at least one movable member is provided for supporting the tank, which can be moved between a first position, in which it supports the tank, and a second position, in which it is disengaged from the tank, causing it to move downwardly. Also provided are a sensor for detecting an impact against the rear part of the motor vehicle and an actuator for moving the movable support member to its second position, causing lowering of the tank, when the sensor detect an impact which is greater than a predetermined magnitude. In this way, the risk of collapse of the tank structure, with a resulting escape of fuel, is avoided in the event of violent impacts.