  We have studied the angular fluctuations in the speed of light with respect
to the apex of the dipole of Cosmic Microwave Background (CMB) radiation using
the experimental data obtained with GRAAL facility, located at the European
Synchrotron Radiation Facility (ESRF) in Grenoble. The measurements were based
on the stability of the Compton edge of laser photons scattered on the 6 GeV
monochromatic electron beam. The results enable to obtain a conservative
constraint on the anisotropy in the light speed variations \Delta c(\theta)/c <
3 10^{-12}, i.e. with higher precision than from previous experiments.
