The European Commission is to proceed with controversial proposals for new rules to class investments in some gas and nuclear power plants as sustainable, which environmental campaigners denounced as a “mockery” of the 27-nation bloc’s claims to climate leadership.

The plans, confirmed on Wednesday, have come under fire from environmental groups, including Greenpeace, and some governments, with European Union countries and investors split over whether the energy should count as green.

“We should accelerate phase-out of more harmful energy sources, move to energy mix mostly based on renewables,” EC Vice President Valdis Dombrovskis said on Twitter.

"But states have different starting points. So under strict conditions, gas & nuclear can act as bridge to a greener energy system as transition activities."

Brussels has been attempting for more than a year to settle whether gas and nuclear energy should be labelled as green in the EU's taxonomy, a rulebook that defines which investments can be marketed as climate-friendly.

Four countries wrote to the EC on Monday urging it to exclude gas, citing a “lack of scientific evidence” for labelling the fuel as green.

Austria and Luxembourg have threatened legal action if the EU brands nuclear as sustainable.

Countries, including France, which gets around 70% of its power from nuclear, say the carbon dioxide-free energy source has a major role to play in addressing climate change, while some central and eastern European countries view gas as needed to quit more-polluting coal.

A super-majority of 20 EU countries — out of the total 27 — or a majority of European Parliament's 700 lawmakers could veto the final rules during a four-month scrutiny period.

The Commission's expert advisers last week said the draft rules on gas and nuclear did not align with the EU's climate change targets, including its goal to have net zero emissions by 2050.

Greenpeace EU sustainable finance campaigner Ariadna Rodrigo said the plans would mean billions of euros would be sunk into technologies that “either do nothing to fight the climate crisis, like nuclear, or which actively make the problem worse, like fossil gas” instead of renewables.

“This anti-science plan represents the biggest greenwashing exercise of all time. It makes a mockery of the EU’s claims to global leadership on climate and the environment,” Rodrigo said.

“The inclusion of gas and nuclear in the taxonomy is increasingly difficult to explain as anything other than a giveaway to two desperate industries with powerful political friends.”

Under the rules, gas power plants would be labelled green this decade if they met an emissions limit of 270 grammes of CO2 equivalent per kilowatt hour, or have annual emissions below 550 kilogrammes CO2e per kilowatt over 20 years.

That could include gas plants with relatively high CO2 emissions today, provided they switch to low-carbon gas or reduce their running hours in later years.

Gas plants must also switch to run on low-carbon gases by 2035.

New nuclear plants must receive construction permits before 2045 to get a green investment label and must be located in a country with a plan and funds to safely dispose of radioactive waste by 2050.

"We're setting out how gas and nuclear could make a contribution in the difficult transition to climate neutrality," EU financial services chief Mairead McGuinness said.

"We're putting in place strict conditions for their inclusion in the taxonomy."

The rules arrive as Europe grapples with surging energy prices and concerns about its reliance on imported Russian gas amid political tensions over Ukraine.