[en] This paper reports on mechanial deformation of the YBa₂Cu₃O_7-δ high temperature superconductor under a number of different processing conditions which resulted in the formation of left-angle 100 right-angle and left-angle 110 right-angle edge dislocations, both having a (001a) slip plane. Subsequent high temperature annealing at 900 degrees C resulted in the formation of extrinsic stacking faults with a large separation of the partial dislocations, up to 0.35μm, suggesting a very low minimum stacking fault energy of 1.2 x 10^-2 J/m². High resolution transmission electron microscopy (HRTEM) in conjunction with image simulations revealed that the stacking faults were comprised of an extra CuO plane between the Ba layers with an offset of b/2. The stacking fault vector of 1/6[031] requires some separation of the left-angle 010 right-angle Burgers vectors into the c-axis direction. A model in which [010] separates into 1/6[031] + 1/6[031] is consistent with the observed stacking faults