The European Commission defied protests from green campaigners and dissent in its own ranks to give a sustainable finance label to investments in both gas and nuclear power on Wednesday.

The EU argues that both have a role to play as cleaner power sources during the transition to a net-zero carbon future.

Environment Minister Leonore Gewessler told reporters that if the EU pressed ahead with its new classification of sustainable energy sources Austria "will file legal action at the European Court of Justice."

She added Luxembourg had already said it would join the legal complaint against the move.

Austrian Chancellor Karl Nehammer too slammed the inclusion of nuclear power on the list of "green" energy sources called the EU Taxonomy.

"Nuclear power is neither 'green' nor sustainable. I cannot understand the decision of the EU," the conservative said on Twitter.

Gewessler described nuclear energy as "outdated" and "too expensive" and highlighted safety concerns and uncertainty over how to deal with nuclear waste.

"The decision is wrong because it endangers the future... We are giving our children a backpack full of problems... It's irresponsible," the Greens politician said.

In an interview last month, Gewessler told AFP that Austria had "very, very strong arguments" and as such she had "great confidence" a complaint at the ECJ could succeed.

The Alpine nation of nine million people has been fiercely anti-nuclear since an unprecedented vote by its population in 1978 prevented its only nuclear plant -- meant to be the first of several -- from starting operations.

Austria aims to obtain all its electricity from renewable resources by 2030, up from more than three-quarters currently.