A theoretical study of the parametric decay instability as occurring in electron Bernstein wave heating experiments and leading to anomalous reflection is performed. The threshold power is calculated for the fundamental instability mode which is proven to have a lower value. It is demonstrated that for the MAST experimental conditions the backscattering parametric decay instability can arise only if the pump power exceeds 80 kW in any microwave beam. It results that the threshold power is roughly inversely proportional to the square of the pump wave frequency. Hence, to increase the instability threshold lowering the pump frequency is suggested.