  Our article is devoted to the study of the rare $B_s\to \phi \ell^+\ell^-$
decay where $\ell=\mu,\tau$. We compute the relevant form factors in the
framework of the covariant quark model with infrared confinement in the full
kinematical momentum transfer region. The calculated form factors are used to
evaluate branching fractions and polarization observables in the cascade decay
$B\to \phi(\to K^+K^-)\ell^+\ell^-$. We compare the obtained results with
available experimental data and the results from other theoretical approaches.
