  Semileptonic $\Lambda_b$ decays are investigated in the framework of the
relativistic quark model based on the quasipotential approach and the
quark-diquark picture of baryons. The decay form factors are expressed through
the overlap integrals of the initial and final baryon wave functions. All
calculations are done without employing nonrelativistic and heavy quark
expansions. The momentum transfer dependence of the decay form factors is
explicitly determined in the whole accessible kinematical range without any
extrapolations or model assumptions. Both the heavy-to-heavy
$\Lambda_b\to\Lambda_c\ell\nu_\ell$ and heavy-to-light $\Lambda_b\to
p\ell\nu_\ell$ decay branching fractions are calculated. The results agree
within error bars with the experimental value of the branching fraction of the
$\Lambda_b\to\Lambda_c^+l^-\bar\nu_l$ decay. From the recent LHCb data on the
ratio of the branching fractions of the heavy-to-light and heavy-to-heavy
semileptonic $\Lambda_b$ decays the ratio of the Cabbibo-Kobayashi-Maskawa
matrix elements $|V_{ub}|/|V_{cb}|$ is obtained. It is consistent with the
corresponding ratio determined from the inclusive $B$ meson decays.
