A bstract We present a new calculation of the K → π semileptonic form factor at zero momentum transfer in domain wall lattice QCD with N f = 2+1 dynamical quark flavours. By using partially twisted boundary conditions we simulate directly at the phenomenologically relevant point of zero momentum transfer. We perform a joint analysis for all available ensembles which include three different lattice spacings ( a = 0.09 – 0.14 fm), large physical volumes ( m π L > 3 . 9) and pion masses as low as 171 MeV. The comprehensive set of simulation points allows for a detailed study of systematic effects leading to the prediction , where the first error is statistical and the second error systematic. The result allows us to extract the CKM-matrix element and confirm first-row CKM-unitarity in the Standard Model at the sub per mille level.