Laser light incident on targets at intensities such that the electron dynamics is ultrarelativistic gives rise to a harmonic power spectrum extending to high orders and characterized by a relatively slow decay with the harmonic number m that follows a power law dependence, m^-p. Relativistic similarity theory predicts a universal value for p = 8/3 up to some cut-off m = m*. The results presented in this work suggest that under conditions in which plasma effects contribute to the emission

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... m, the extent of this contribution may invalidate the concept of universal decay. We report a decay with harmonic number in the ultrarelativistic range characterised by an index 5/3 < p < 7/3, significantly weaker than that predicted by the similarity model.