Over the past years a lot of theoretical and experimental effort has been made to find states with broken time-reversal symmetry (BTRS) in multiband superconductors. In particular, it was proposed theoretically that in the Ba1-x K-x Fe2As2 system either an s + is or an s + id BTRS state may exist at high doping levels in a narrow region of the phase diagram. Here we report the observation of an enhanced zero-field muon spin-relaxation rate below the superconducting transition temperature for a high quality crystalline sample with x approximate to 0.73. This indicates that indeed the time-reversal symmetry is broken in superconducting Ba1-x K-x Fe2As2 at this doping level.