No Dutch Royals may hold a position of power or public authority in Belgium.
A decree of November 24, 1830 debars members of the House of Orange-Nassau (basically: the Dutch royal family) from any position of power or public authority in Belgium — forever! In theory it not only excludes members of said House from the throne, but also from access to the public sector and from standing for elections.
The decree is considered to have constitutional status. It stems from the period immediately following the rule of the Dutch King William I in what is now Belgium, from 1815 until 1830.