The behaviour of high energy runaway electrons during the flat-top phase of electron cyclotron (EC) resonance heated (ECRH) discharges has been analysed in the Frascati Tokamak Upgrade. The decay of the electric field due to plasma heating has been observed to result in a rapid fall in the electron energy that frequently leads to quenching of the runaway population. The efficiency of this runaway mitigation scheme and its dependence on the plasma conditions has been investigated with particular emphasis on the required electric field drop during EC wave injection.