  Recently it has been argued that tree-level scattering amplitudes in N=4
Yang-Mills theory are uniquely determined by a careful study of their
superconformal and Yangian symmetries. However, at one-loop order these
symmetries are known to become anomalous due to infrared divergences. We
compute these one-loop anomalies for amplitudes defined through dimensional
regularisation by studying the tree-level symmetry transformations of the
unitarity branch cuts, keeping track of the crucial collinear terms arising
from the holomorphic anomaly. We extract the superconformal anomalies and show
that they may be cancelled through a universal one-loop deformation of the
tree-level symmetry generators which involves only tree-level data.
Specialising to the planar theory we also obtain the analogous deformation for
the level-one Yangian generator of momentum. Explicit checks of our one-loop
deformation are performed for MHV and the 6-point NMHV amplitudes.
