We have studied the correlations between the width of the U-shaped peak flux density spectra of solar radio bursts and the slope of the associated proton energy spectra observed by satellites. We find the wider radio spectra U's lead to shallow proton energy slopes (harder spectrum), while narrower U's lead to steeper slopes (softer spectrum). Out of the straight line, power law, and exponential forms used to study the correlations, the power law form yields the best correlation (r?0.77). This leads to a practical prediction scheme which can be used in real time for forecasting the spectral character of the protons that can be expected to arrive in the vicinity of the earth on the basis of the continuously monitored radio burst data.