  The perturbative QCD description of high-energy hadroproduction of two hard
jets separated by a large rapidity gap void of emission (also called
Mueller-Tang jets) is based on a factorization formula of BFKL type that
represents exchanges of colour-singlet objects among the external particles.
This formula resums to all perturbative orders a certain class of Feynman
diagrams that are supposed to dominate the cross-section in the Regge limit.
However, the explicit calculations at next-to-leading logarithmic order
questions the validity of such factorization when an IR safe jet algorithm is
used to reconstruct jets. We show the origin of such violation of
factorization, and quantify its impact for LHC phenomenology. In this
connection, we estimate the impact of other contributions to the cross-section
that are not included in the Mueller-Tang factorization formula -- colour
non-singlet exchanges -- that, at low rapidity separation, compete with the
singlet ones.
