The on-shell massless planar double box diagram with an irreducible numerator

Using a Mellin-Barnes representation, we compute the on-shell massless planar double box Feynman diagram with an irreducible scalar product of loop momenta in the numerator. This diagram is needed in calculations of two loop corrections to scattering processes of massless particles, together with the double box without numerator calculated previously by Smirnov. We verify the poles in epsilon of our result by means of a system of differential equations relating the two diagrams, which we present in an explicit form. We verify the finite part with an independent numerical check.