  We report the detection of an orbital period decay of $\dot P=-1.83\pm0.66$
ms yr$^{-1}$ in the black hole X-ray binary XTE J1118+480. This corresponds to
a period change of $-0.85\pm0.30$ $\mu $s per orbital cycle, which is $\sim
150$ times larger than expected from the emission of gravitational waves. These
observations cannot be reproduced by conventional models of magnetic braking
even when including significant mass loss from the system. The spiral-in of the
star is either driven by magnetic braking under extremely high magnetic fields
in the secondary star or by a currently unknown process, which will have an
impact on the evolution and lifetime of black hole X-ray binaries.
