  A simple flux reconstruction for finite element solutions of
reaction-diffusion problems is shown to yield fully computable upper bounds on
the energy norm of error in an approximation of singularly perturbed
reaction-diffusion problem. The flux reconstruction is based on simple,
independent post-processing operations over patches of elements in conjunction
with standard Raviart--Thomas vector fields and gives upper bounds even in
cases where Galerkin orthogonality might be violated. If Galerkin orthogonality
holds, we prove that the corresponding local error indicators are locally
efficient and robust with respect to any mesh size and any size of the reaction
coefficient, including the singularly perturbed limit.
