Environmental campaign groups have submitted a legal complaint to the European Commission, the EU executive, for failing to implement a proposal that labels Canada's tar sands and other non-conventional fuel as highly polluting.
If implemented, the draft law would slow Europe's imports of tar sands crude, which scientists say is especially harmful to the environment. Imports have so far been limited, but are predicted to increase as new pipelines come on stream.
Directors of Greenpeace, Transport & Environment, and Friends of the Earth Europe sent their complaint on Thursday to senior Commission officials.
If the Commission fails to respond, the campaign groups say they will take the issue to the top EU courts in Luxembourg.
The Commission was not immediately available for comment.
The campaigners' nine-page submission says the Commission has violated its statutory obligations to act without delay and requests an internal review of that "administrative omission".
Failure to implement the law would represent a major setback for EU climate policy, it says.
"It would be - and be seen as - a victory for those companies and countries that aim to sell high-carbon fuels to Europe and have chosen to ignore the climate change impact of their activities," the formal submission says.
For years, debate has raged over a draft EU law that ranks transport fuels according to how much carbon they emit over their life cycle to guide EU member states seeking to meet 2020 targets on cutting emissions.
Tar sands crude, also known as oil sands, requires large amounts of energy to extract, making it more carbon-intensive than conventional oil.
Opposition to the EU law has been led by Canada.
The top producer of unconventional oil is waiting for the final word from U.S. PresidentBarack Obama on its planned Keystone XL pipeline, part of the Keystone Pipeline System to transport oil from Canada to U.S. Gulf Coast refineries. Oil products refined there are subsequently shipped to Europe.
An environmental review from the U.S. State Department is expected to say theTransCanada Corp project will not appreciably increase carbon emissions, adding pressure on Obama to take a decision.
To address the objections to the proposed EU law, known as the Fuel Quality Directive, the European Commission said in April 2012 it was carrying out a full assessment into the impact of its proposal, but has yet to make the results public.
A package of guidance this month on 2030 energy policy to follow on from existing 2020 climate and energy targets omitted any specific targets on cutting emissions from transport, the fastest-growing source of greenhouse gases.
Campaigners say that means the Fuel Quality Directive is effectively being killed off before it has even begun to be used to limit the carbon impact of fuel-burning.