Hours before the window for lodging objections closes, EU environment and energy ministers meeting in France Friday differed sharply on a European Commission provision that would classify nuclear and natural gas energy as "sustainable".

The controversy pits countries led by France -- where nuclear generates a world-leading 70 percent of electricity -- against Germany, Austria and others in the 27-nation bloc.

Debate over the Commission's so-called "taxonomy" is not on the agenda of the informal, three-day talks in Amiens, but flared nonetheless.

In late December the European Commission unveiled a classification labelling investment in nuclear gas-based energy as sustainable, in order to favour sectors that reduce the greenhouse gas emissions driving global warming.

Nuclear power is carbon-free, and gas is significantly less polluting than coal.

Countries in the European Union had until midnight Friday to suggest modifications.

After that, the Commission -- taking these suggestions into account -- must "rapidly" publish a final text that will be definitely adopted four months later.

Passage in its current form seems more than likely: it would take a majority of deputies in the EU parliament or 20 of the 27 members states to derail it, and critical mass is lacking in both cases.

A letter to the executive European Commission from some European Parliament deputies protesting that the period for suggesting changes was too short has fallen on deaf ears.

And among EU member states, a dozen have backed France's position and the Commission's proposed taxonomy.

Many are central European nations looking to switch from carbon-intensive coal-fired power to natural gas.

"Nuclear is a decarbonised energy," French environment minister Barbara Pompili told journalists in Amiens.

"We cannot deprive ourselves of it at the same time that we need to very rapidly reduce our carbon emissions."

- 'A very bad signal' -

Despite the strong headwinds, anti-nuclear resistance has not subsided.

"It is neither sustainable nor economic", countered Germany environment minister Stefan Tidow. "It is not a green energy."

Luxembourg and Austria have gone even further, threatening to take the case to court if nuclear is certified as sustainable, citing the risk of accidents and the as-yet unresolved problem of nuclear waste.

"It would be greenwashing," Luxembourg's environment minister, Carole Dieschbourg, told AFP.

"And it would send a very bad signal: it is not a transition energy, it takes too long," she added, alluding to the lag time for building nuclear reactors.

Her Austrian counterpart, Leonore Gewessler, said labelling nuclear power as sustainable will "undermine the credibility of the taxonomy" because it does not fulfil the legal criterion of "not causing damage to the environment".

The EU Commission has proposed a measure requiring financial products to specify what percentage of the activities financed involve nuclear energy, a transparency measure that would allow investors to steer clear if they wanted to.

Berlin has expressed reservations about joining Vienna and Luxembourg in a legal challenge.

"For now, we're working on our response, and when the Commission presents a new text we'll analyse it from a legal standpoint," said Germany state secretary for economic affairs and climate action Sven Giegold.

Austria has also objected to tagging gas as sustainable, with The Netherlands -- which backs the label for nuclear energy -- arguing "there is no scientific reason to include" gas.

Polish undersecretary of state for the environment Adam Guibourge-Czetwertynski disagreed.

"Gas replacing coal because there's nothing better in the short term, that makes sense," he said.