ONE Of the long-standing mysteries associated with the high-temperature copper oxide superconductors concerns the anomalous suppression(1) of superconductivity in La2-xBaxCuO4 (and certain related compounds) when the hole concentration x is near 1/8. Here we examine the possibility that this effect is related to dynamical two-dimensional spin correlations, incommensurate with the crystal lattice, that have been observed in La2-xSrxCuO4 by neutron scattering(2-4). A possible explanation for the incommensurability involves a coupled, dynamical modulation of spin and charge in which antiferromagnetic ’stripes’ of copper spins are separated by periodically spaced domain walls to which the holes segregate(5-9). An ordered stripe phase of this type has recently been observed in hole-doped La2NiO4 (refs 10-12). We present evidence from neutron diffraction that in the copper oxide material La1.6-xNd0.4SrxCuO4, with x=0.12, a static analogue of the dynamical stripe phase is present, and is associated with an anomalous suppression of superconductivity(13,14). Our results thus provide an explanation of the ’1/8’ conundrum, and also support the suggestion(15) that spatial modulations of spin and charge density are related to superconductivity in the copper oxides.