  We present a new fast algorithm which allows the simulation of ionising
radiation emitted from point sources to be included in high-resolution
three-dimensional smoothed particle hydrodynamics simulations of star cluster
formation. We employ a Str\"omgren volume technique in which we use the
densities of particles near the line-of-sight between the source and a given
target particle to locate the ionisation front in the direction of the target.
Along with one--dimensional tests, we present fully three--dimensional
comparisons of our code with the three--dimensional Monte-Carlo radiative
transfer code, MOCASSIN, and show that we achieve good agreement, even in the
case of highly complex density fields.
