  We present an all-order generalized factorization formula for QCD scattering
amplitudes in kinematical configurations where two or more momenta of the
external partons become collinear. The singular behaviour of the scattering
amplitudes in the collinear limit is encoded by collinear splitting matrices.
In the space-like region and beyond the tree level, the collinear splitting
matrices depend also on the momenta and quantum numbers of the non-collinear
partons, thus breaking strict collinear factorization. Although the
factorization breaking contribution partly cancels for squared amplitudes, due
to its one-loop absorptive origin, remaining effects at high perturbative
orders have implications on the non-abelian structure of
logarithmically-enhanced terms in perturbative calculations and on various
factorization issues of mass singularities.
