A system and a method for non-intrusive and continuous level measurement of a liquid enclosed by a solid wall of a vessel. The system comprises an ultrasonic transmitter for generating an ultrasound wave and for emitting it into the vessel wall, an ultrasonic receiver for receiving the ultrasound wave, and an electronic control and data processing unit (ECDU) for controlling operation of the transmitter and of the receiver and for determining the liquid level. The transmitter is able to emit the ultrasound wave as a primary Lamb wave into the vessel wall so that a part of the primary Lamb wave leaks from the vessel wall into the liquid in form of a pressure wave. The ECDU is adapted to repeatedly determine the time of flight of the pressure wave, change the ultrasonic frequency of the transmitter until the determined time of flight reaches a minimum, and determine the liquid level.