In general relativity gravitational waves propagate at the speed of light , however in alternative theories of gravity that might not be the case . We study the effects of a modified speed of gravity , c _ { T } ^ { 2 } , on the B-modes of the Cosmic Microwave Background ( CMB ) anisotropy in polarisation . We find that a departure from the light speed value would leave a characteristic imprint on the BB spectrum part induced by tensors , manifesting as a shift in the angular scale of its peaks . We derive constraints by using the available Planck and BICEP2 datasets showing how c _ { T } ^ { 2 } can be measured , albeit obtaining weak constraints due to the overall poor accuracy of the current BB power spectrum measurements . The present constraint corresponds to c _ { T } ^ { 2 } = 1.30 \pm 0.79 and c _ { T } ^ { 2 } < 2.85 at 95 \% C.L . by assuming a power law primordial tensor power spectrum and c _ { T } ^ { 2 } < 2.33 at 95 \% C.L . if the running of the spectral index is allowed . We derive forecasts for the next generation CMB satellites , which we find capable of tightly constraining c _ { T } ^ { 2 } at percent level , comparable with bounds from binary pulsar measurements , largely due to the absence of degeneracy with other cosmological parameters .