Brussels and NGOs expressed concerns Tuesday about several EU countries, including Germany, reverting to using coal for power generation as the fall-out from Russia's war in Ukraine hits energy supplies.
"We have to make sure that we use this crisis to move forward and not to have a backsliding on the dirty fossil fuels," European Commission chief Ursula von der Leyen told several European media in an interview.
"It's a fine line and it is not determined whether we are going to take the right turn," she added.
The shift -- a reaction to power-hungry Europe being increasingly starved of Russian gas and oil -- seriously undermines the EU's vaunted ambition to become climate neutral by 2050.
That goal is one of the cornerstones of von der Leyen's policies at the helm of the EU executive.
Germany, Austria and the Netherlands have in the past couple of days said they will ease restrictions on power stations fired by fossil fuel.
German Economy Minister Robert Habeck on Tuesday described Russian energy giant Gazprom's slashing of gas supplies to Europe as an "attack on us" by Moscow.
While Germany, Europe's biggest economy and the region's biggest energy consumer, said it still plans to exit coal in 2030, environmental groups are sceptical.
Turning back to coal "is a bad choice" with structural consequences, said Neil Makaroff, of Climate Action Network, an umbrella organisation for such groups.
"Countries are continuing to back fossil energy rather than investing enough in renewables," he said.
"The risk is substituting one dependency for another: importing Colombian or Australian coal, US or Qatari liquified natural gas, to replace Russian hydrocarbons."
Another group, Carbon Market Watch, agreed that the move to coal was "worrying" and expressed hope it would "be as temporary as possible".
The European Union, as part of swingeing sanctions imposed on Russia for its invasion of Ukraine, is phasing in a ban on Russian coal and oil.
Moscow, in turn, has taken to turning down gas supplies to EU countries.
Although it says the diminished supplies are because of technical or maintenance reasons, European capitals believe Russia is trying to hurt the EU for its backing of Ukraine, in particular its candidacy bid to one day join the bloc.