Understanding the nature of the ground state and its low-lying excitations in the copper oxide superconductors is a prerequisite for determining the origin of high-temperature superconductivity. A superconducting order parameter (that is, the energy gap) with a predominantly dx2-y2 symmetry is well-established. However, various deviations from a pure d-wave pair state, such as the possibility of Cooper pairing with broken time-reversal symmetry or an admixed dx2-y2+s pair state, have been theoretically predicted and actively sought in numerous experimental studies. Here, we present an angle-resolved phase-sensitive technique for accurately determining the in-plane pairing symmetry, and demonstrate this technique in optimally doped YBa2Cu3O7-δ. We find that the gap along the b-axis (Cu-O chain) direction is at least 20% larger than that along the a-axis direction, and that any imaginary idxy, is or ip component must be smaller than a few per cent of the dx2-y2 component of the gap.