  We present and analyze a preconditioner of the additive Schwarz type for the
mortar boundary element method. As a basic splitting, on each subdomain we
separate the degrees of freedom related to its boundary from the inner degrees
of freedom. The corresponding wirebasket-type space decomposition is stable up
to logarithmic terms. For the blocks that correspond to the inner degrees of
freedom standard preconditioners for the hypersingular integral operator on
open boundaries can be used. For the boundary and interface parts as well as
the Lagrangian multiplier space, simple diagonal preconditioners are optimal.
Our technique applies to quasi-uniform and non-uniform meshes of shape-regular
elements. Numerical experiments on triangular and quadrilateral meshes confirm
theoretical bounds for condition and MINRES iteration numbers.
