An LNG fuel tank system for at least one gas engine used for ship propulsion is comprising at least one LNG fuel tank (4) and a gas vessel (8), the LNG fuel tank to be-bunkered from an onshore LNG pressure tank filling facility by means of an LNG filling line (1). According to the present invention the LNG fuel tank (4) is a ship low pressure controlled atmospheric pressure LNG tank, and the gas vessel (8) is a single shell non-insulated pressure vessel arranged to accumulate flashed and boil-off gas during LNG bunkering and pressure relieving the LNG fuel tank, respectively, and the gas engines are fuelled from either the gas vessel (8) or the LNG fuel tank (4), dependent on a predefined gas vessel pressure.