In 2015, the Flemish government introduced a new tax intended to help cover the deficit caused by green energy certificates. This controversial tax was nicknamed the Turteltax, a reference to then Energy Secretary Annemie Turtelboom. The tax, which entered into effect on 1 March 2016, added EUR 100,- on average to the annual electricity bill of Flemish households.

The consumer organisation Test-Aankoop, assisted by NautaDutilh's Patrick Peeters and Julie Vanhoenacker, brought proceedings before the Constitutional Court seeking to overturn the tax. Test-Aankoop argued that the decree introducing the Turteltax violated Article 170 §2 of the Constitution (and its implementing legislation), which provides that a taxpayer cannot be subjected to identical federal and regional taxes. Indeed, the Flemish legislature is taxing the consumption of electricity despite the fact that a federal electricity tax already exists, hence there is double taxation.

In a judgment (no. 83/2017) of 22 June 2017, the Constitutional Court sided with Test-Aankoop and set aside the Turteltax decree. However, the Constitutional Court maintained the effects of the invalidated provisions of the decree with regard to 2016 and 2017 in order to avoid legal uncertainty and administrative difficulties. Consumers are therefore not entitled to a reimbursement of the Turteltax for those years. However, as from 2018, the tax can no longer be imposed.