The groups are targeting the EU's "taxonomy", a list of investments that can be labelled and marketed as sustainable in Europe.
Greenpeace and other campaign groups are set to take the European Commission to court on Tuesday, seeking to overturn European Union rules that class nuclear energy and natural gas as climate-friendly investments.
The groups are targeting the EU's "taxonomy" - a list of investments that can be labelled and marketed as sustainable in Europe. The complex scheme aims to guide investors towards projects that support the EU's climate change goals.
The Commission decided last year to add some gas and nuclear plants to the list - a proposal that became mired in controversy and was repeatedly delayed amid lobbying from governments who disagree on the fuels' green credentials.
In its lawsuit, Greenpeace asks the EU General Court - part of the Court of Justice of the EU - to exclude gas and nuclear from the rules.
It argues the EU has violated its own climate laws, citing the CO2 emissions produced by gas power plants, and said the rules risk diverting investments away from renewable energy.
The EU is facing more than one lawsuit
A separate lawsuit was filed on Tuesday, from four campaign groups including environmental legal charity ClientEarth and WWF, seeking to overturn the rules on gas on similar grounds.
"Fossil gas is not clean, not cheap and not a secure source of energy," a spokesperson for the four groups said.
They add that the taxonomy is meant to support the EU in meeting its climate targets. Instead, the groups claim this label stands to do the opposite by channelling additional investments into this "harmful energy source".
"We're taking the Commission to court in the hope of restoring some credibility to the taxonomy and avoiding this huge risk to climate and people's energy security."
Their desired outcome would be a ruling that forces the Commission to review the Complementary Delegated Act - the legislation that classifies nuclear and gas as 'green'.
What does the European Commission have to say?
A European Commission spokesperson said the EU executive took note of the legal action but declined to comment further.
The Commission has previously said gas and nuclear plants must meet "strict conditions" to win the EU green label, including an emissions limit for gas plants.
The EU rules exposed deep rifts between countries over which energy sources to use to meet climate change goals.
Spain, Denmark and others had argued it was not credible to label gas, a CO2-emitting fossil fuel, as climate-friendly. Poland, Bulgaria and others said gas investments were needed to help them phase out more CO2-intensive coal plants.
The Commission is also facing a legal challenge from the Austrian government, seeking to reject the green label for gas and nuclear.