  We consider a conforming finite element approximation of the Reissner-Mindlin
system. We propose a new robust a posteriori error estimator based on H(div)
conforming finite elements and equilibrated fluxes. It is shown that this
estimator gives rise to an upper bound where the constant is one up to higher
order terms. Lower bounds can also be established with constants depending on
the shape regularity of the mesh. The reliability and efficiency of the
proposed estimator are confirmed by some numerical tests.
