  We consider the singular behaviour of one-loop QCD matrix elements when
several external partons become simultaneously parallel. We present a new
factorization formula that describes the singular collinear behaviour directly
in colour space. The collinear singularities are embodied in
process-independent splitting matrices that depend on the momenta, flavours,
spins and colours of the collinear partons. We give the general structure of
the infrared and ultraviolet divergences of the one-loop splitting matrices. We
also present explicit one-loop results for the triple collinear splitting, $q
\to q {\bar Q} Q$, of a quark and a quark--antiquark pair of different
flavours. The one-loop triple collinear splitting is one of the ingredients
that can be used to compute the evolution of parton distributions at the
next-to-next-to-leading order in QCD perturbation theory.
