Such interests range from a seat on the local council or membership of the board of a social housing cooperative to executive positions on the board of a multi-national company.
However, each year around 100 politicians and civil servants fail to provide the necessary information.
Most offenders say that they simply forgot to make a declaration.
However, some like the Mayor of the West Flemish municipality of Deerlijk Claude Croes (Christian democrat) do it deliberately.
Last year Mr Croes said that he had not made a declaration as “The powers that be know how much everyone earns anyway.”
The names of the politicians and civil servants that face prosecution have not been given.
The 2004 law on the declaration of outside interest received near-unanimous backing in the Chamber of Representatives and the Senate.
The late Flemish liberal MP Pierre Lano was the only member of the Chamber to abstain.