  We study the spontaneous excitation of a two-level atom in the presence of a
perfectly reflecting mirror, when the atom, or the mirror, is uniformly
accelerating in the framework of the generalised uncertainty principle (GUP).
The quantized scalar field obeys a modified dispersion relation leading to a
GUP deformed Klein-Gordon equation. The solutions of this equation with
suitable boundary conditions are obtained to calculate the spontaneous
excitation probability of the atom for the two separate cases. We show that in
the case when the mirror is accelerating, the GUP modulates the spatial
oscillation of the excitation probability of the atom, thus breaking the
symmetry between the excitation of an atom accelerating relative to a
stationary mirror, and a stationary atom excited by an accelerating mirror. An
explicit violation of the equivalence principle is demonstrated. We further
obtain an upper bound on the GUP parameter using standard values of the system
parameters.
