A method of force estimation for a minimally invasive medical system comprising a robot manipulator (10). The manipulator has an effector unit (12) equipped with a 6-degrees-of-freedom (DOF) force/torque sensor and is configured to hold a minimally invasive instrument (14) having a first end (16) mounted to the effector unit and a second end (20) located beyond an external fulcrum (23) that limits the instrument in motion, usually to 4 DOF. The method comprising the steps: --determining a position of the instrument relative to the fulcrum; --measuring by means of the 6-DOF force/torque sensor a force and a torque exerted onto the effector unit by the first end of the instrument; and --calculating by means of the principle of superposition an estimate of a force exerted onto the second end of the instrument based on the determined position, the measured force and the measured torque.