  A mathematical analysis of the distribution of voting power in the Council of
the European Union operating according to the Treaty of Lisbon is presented. We
study the effects of Brexit on the voting power of the remaining members,
measured by the Penrose--Banzhaf Index. We note that the effects in question
are non-monotonic with respect to voting weights, and that some member states
will lose power after Brexit. We use the normal approximation of the
Penrose--Banzhaf Index in double-majority games to show that such
non-monotonicity is in most cases inherent in the double-majority system, but
is strongly exacerbated by the peculiarities of the EU population vector.
Furthermore, we investigate consequences of a hypothetical "generalized
Brexit", i.e., NN-exit of another member state (from a 28-member Union), noting
that the effects on voting power are non-monotonic in most cases, but strongly
depend on the size of the country leaving the Union.
