Opposing viewpoints on whether or not fossil gas should be regarded as sustainable energy will be decided in court.
Under the European Union's sustainable finance Taxonomy (Complementary Delegated Act), certain uses of gas are deemed environmentally sustainable.
But four non-governemental organisations disagree.
"Fossil gas is a high-carbon source of energy when burnt and, when leaked into the atmosphere, its impact on climate change is more than 80 times greater than CO2 over 20 years," an ngo statement said.
The ngos will argue in court that the CDA clashes with EU laws such as the Taxonomy Regulation itself and the European Climate Law. It also goes against the EU's obligations under the Paris Agreement.
The court action has been brought by Transport & Environment, ClientEarth, WWF European Policy Office and BUND (Friends of the Earth Germany).