  In solar flares, a large number of charged particles is accelerated to high
energies. By which physical processes this is achieved is one of the main open
problems in solar physics. It has been suggested that during a flare, regions
of the rapidly relaxing magnetic field can form a collapsing magnetic trap
(CMT) and that this trap may contribute to particle energisation.} In this
Research Note we focus on a particular analytical CMT model based on kinematic
magnetohydrodynamics. Previous investigations of particle acceleration for this
CMT model focused on the non-relativistic energy regime. It is the specific aim
of this Research Note to extend the previous work to relativistic particle
energies. Particle orbits were calculated numerically using the relativistic
guiding centre equations. We also calculated particle orbits using the
non-relativistic guiding centre equations for comparison. For mildly
relativistic energies the relativistic and non-relativistic particle orbits
mainly agree well, but clear deviations are seen for higher energies. In
particular, the final particle energies obtained from the relativistic
calculations are systematically lower than the energies reached from the
corresponding non-relativistic calculations, and the mirror points of the
relativistic orbits are systematically higher than for the corresponding
non-relativistic orbits. While the overall behaviour of particle orbits in CMTs
does not differ qualitatively when using the relativistic guiding centre
equations, there are a few systematic quantitative differences between
relativistic and non-relativistic particle dynamics.
