We consider hybrid inflation for small couplings of the inflaton to matter such that the critical value of the inflaton field exceeds the Planck mass .
It has recently been shown that inflation then continues at subcritical inflaton field values where quantum fluctuations generate an effective inflaton mass .
The effective inflaton potential interpolates between a quadratic potential at small field values and a plateau at large field values .
An analysis of the allowed parameter space leads to predictions for the scalar spectral index n _ { s } and the tensor-to-scalar ratio r similar to those of natural inflation .
Using the ranges for n _ { s } and r favoured by the Planck data , we find that the energy scale of the plateau is constrained to the interval ( 1.6 - 2.4 ) \times 10 ^ { 16 } \mathrm { GeV } , which includes the energy scale of gauge coupling unification in the supersymmetric standard model .
The tensor-to-scalar ratio is predicted to satisfy the lower bound r > 0.049 for 60 e -folds before the end of inflation .