Abstract. We put forward a method of separating the geomagnetic activity contribution to the F2-layer critical frequency median, f o F2 med , at middle latitudes. It is based on the analysis of δf o F2, which is the ratio f o F2 med /f o F2 q in percent, where f o F2 q is the F2-layer critical frequency for quiet conditions. The quantities f o F2 q and δf o F2 depend on solar and geomagnetic activity respectively. These dependences are taken into account using indices F 12 (annual average solar radio emission flux at 10.7 cm) and Ap m (monthly average Ap index), thus facilitating the use of this method for forecasting f o F2 med . With this method, from Slough station (51.5° N, 0.6° W) data for midday and midnight for 1954 to 1995 we have found that at midnight the δf o F2 dependence on Ap m is significant at the 95 % confidence level for equinoxes and summer. For midday, this dependence is less pronounced and is significant only from April to July. At equinoxes and summer, an Ap m increase causes a δf o F2 decrease. For midnight, this feature is more pronounced than for midday. This regularity is also valid for annual average Ap m and δf o F2.