A method of in line verification of a flow meter installed in a flow line by means of one or two ultrasonic clamp on flow meters to be installed in line with the flow meter uΔ, comprising a preparatory phase, during which a virtual pipe diameter is determined, which if it had been applied during the simultaneous measurements would have minimized the differences between the measured flow indications of the flow meter and the measured flow indications, and comprising a verification phase, during which the ultrasonic flow meters are set up to determine their measured flow indications based on the virtual pipe diameter, during which simultaneously derived measured flow indications of the flow meter and the ultrasonic flow meters are recorded, and during which an alert is issued in case a deviation between simultaneously derived measured flow indications of the flow meter and the ultrasonic flow meter or between measured flow indications of the flow meter and averages of the measured flow indications of the ultrasonic flow meters respectively exceeds a predetermined deviation limit, which was set based on a measurement uncertainty inherent to the determination of the deviation.