  We prove that the celebrated It\^{o}'s theorem for groups remains valid at
the level of Leibniz algebras: if $\mathfrak{g}$ is a Leibniz algebra such that
$\mathfrak{g} = A + B$, for two abelian subalgebras $A$ and $B$, then
$\mathfrak{g}$ is metabelian, i.e. $[ \, [\mathfrak{g}, \, \mathfrak{g}], \, [
\mathfrak{g}, \, \mathfrak{g} ] \, ] = 0$. A structure type theorem for
metabelian Leibniz/Lie algebras is proved. All metabelian Leibniz algebras
having the derived algebra of dimension $1$ are described, classified and their
automorphisms groups are explicitly determined as subgroups of a semidirect
product of groups $P^* \ltimes \bigl(k^* \times {\rm Aut}_{k} (P) \bigl)$
associated to any vector space $P$.
