We report on the generation of a narrow divergence (theta(gamma) <2.5 mrad), multi-MeV (E-max approximate to 18 MeV) and ultrahigh peak brilliance (> 1.8 x 10(20) photons s(-1) mm(-2) mrad(-2) 0.1% BW) gamma-ray beam from the scattering of an ultrarelativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude a(0) approximate to 2). The spectrum of the generated gamma- ray beam is measured, with MeV resolution, seamlessly from 6 to 18 MeV, giving clear evidence of the onset of nonlinear relativistic Thomson scattering. To the best of our knowledge, this photon source has the highest peak brilliance in the multi-MeV regime ever reported in the literature.