[en] A method is proposed for estimating ratio fluctuations of the peak power to the duration of ultrashort pulses (USPs), based on the measurement of the radiation spectrum nonlinearly broadened in the Kerr medium for only four wavelengths. The method provides a fairly simple measurement processing which can be performed in real time using analogue circuitry even without the digitisation of signals, whereas the operation speed is only limited by the response time of photodetectors and analogue circuits. Numerical studies have shown the possibility of applying the method even for pulses having a substantial asymmetry and containing a significant pedestal. The use of the developed method for analysis of fluctuations of the pulse parameters of a high-power laser system has been experimentally demonstrated, with a short optical fibre section employed for spectrum broadening. (paper)