A new method of the optimal forecasting of the peak of the following 11-year activity cycle up to its beginning and several subsequent cycles of the activity is developed. The method is based on the regularity established earlier, namely, long-time cyclical variations of the activity, radius and solar constant, being a result of the same processes, are correlated both on the phase and amplitude. The most probable peak of the solar activity cycle 24 is W_max = 70±10 units of the sunspot number. In the subsequent activity cycles 25—26 which will be formed during the decay of the present centenary solar activity cycle the tendency for the drop of the peak of the activity cycle up to W_max = 50±15 and 35±20 units of the sunspot number, respectively, will be kept. 11-year activity cycle,solar radius