  We compute the one-loop four-point function in {\cal N}=4 supersymmetric
Yang-Mills theory with gauge group U(N). We perform the calculation in {\cal
N}=1 superspace using the background field method and obtain the complete
off-shell contributions to the effective action from planar and non planar
supergraphs. In the low-energy approximation the result simplifies and we can
study its properties under gauge transformations. It appears that the nonplanar
contributions do not maintain the gauge invariance of the classical action.
