  We present a model for the formation of high-mass close binary systems in the
context of forming massive stars through gas accretion in the centres of
stellar clusters. A low-mass wide binary evolves under mass accretion towards a
high-mass close binary, attaining system masses of order 30-50 solar masses at
separations of order 1 AU. The resulting high frequency of binary systems with
two massive components is in agreement with observations. These systems are
typically highly eccentric and may evolve to have periastron separations less
than their stellar radii. Mergers of these binary systems are therefore likely
and can lead to the formation of the most massive stars, circumventing the
problem of radiation pressure stopping the accretion. The stellar density
required to induce binary mergers is one million stars per cubic parsec, or
approximately one percent that required for direct stellar collisions.
