  We introduce a new asymmetry in the decay t -> W b -> l nu b, which is shown
to be directly proportional to the polarisation of the top quark along a chosen
axis, times a sum of W helicity fractions. The latter have already been
precisely measured at the Tevatron and the Large Hadron Collider. Therefore,
this new asymmetry can be used to obtain a model-independent measurement of the
polarisation of top quarks produced in any process at hadron or lepton
colliders.
