CP violation in a light Higgs boson decay from $\tau$-spin correlations at a linear collider

10 pages, 4 figuresWe present a new method to measure the transverse spin correlation in the $H\to\tau^+\tau^-$ decay. The method has been devised to be insensitive to the beamstrahlung which affects the definition of the beam energy at a linear collider. In the case of two $\tau\to\pi^\nu$ decays, using the anticipated detector performance of the TESLA project, we get a promising estimation of the error expected on the measurement of a CP violating phase