Government sources in Berlin said Germany would not support the deal reached early this week to enforce a new CO2 limit of 95 grams per kilometre (g/km) as an average across the EU fleet.
In Brussels three sources, speaking on condition of anonymity, said Germany was seeking allies to overturn the provisional agreement and was applying intense pressure on fellow member states.
Representatives of member states are expected to meet on Thursday to consider the deal.
"The Germans, at the highest possible level, are piling on a lot of pressure," one of the sources said.
Diplomatic sources told EurActiv that German Chancellor Angela Merkel was up late last night lobbying heads of government to back her in delaying the vote.
A draft agreement late on Monday allowed for some flexibility in enforcement of the new 2020 standard, but less than Germany had hoped for.
The EU's rotating Irish presidency clearly backed the deal, saying that it struck the right balance between environmental ambition and economic considerations. But Enda Kenny, the Irish premier, reportedly took the deal off the agenda of the Council following pressure from Merkel.
Germany has been lobbying for weeks to shelter its premium car sector by campaigning for loopholes, known as supercredits.
These allow manufacturers to carry on producing more polluting vehicles provided they also make very low emissions vehicles, such as electric cars.
Germany says supercredits encourage innovation, while the Commission says too many of them mean producers can carry on making higher emissions models and emissions levels will fail to meet the target of 95 g/km target by 2020.
One German carmaker, speaking on condition of anonymity, called the cars deal "a victory for the southern Europeans," meaning Italy, Spain and France, as opposed to Germany.
"The car industry provides a lot of jobs in Germany and is a pillar of Germany's competitiveness," a government source said.
In parallel with the cars debate, the EU has also worked on new rules for vans and those were agreed on Wednesday, although they, like the car proposals, still need official endorsement.
The agreement on vans sets a limit of 147 g/km as an average for vehicles across the EU fleet from 2020, a spokeswoman for the EU presidency, held by Ireland until the end of June, said.
The European Automobile Manufacturers' Association (ACEA) - which represents vehicle makers including Daimler, Ford of Europe and General Motors Europe - has said both sets of draft law are extremely ambitious and upfront investment in new technology is a burden in difficult times.
Environmental campaigners say increased initial costs have been exaggerated and for consumers are quickly offset by fuel savings over the life-time of a car or van.
They say the vans target is particularly weak, even taking account of their greater weight, although it is accompanied by a declaration from the European Commission on the need to continue to cut vans' emissions beyond 2020 and does not include any supercredits.
Under the rules for both cars and vans, each manufacturer is assigned an individual target to take account of the nature of their fleet and their past cuts.
Independent research has backed up arguments from the Commission that moving towards more fuel-efficient cars can help to ensure the EU industry is a world leader in innovation, can generate jobs and cut fuel bills. In turn, that can stimulate consumer spending elsewhere and spur economic growth.
Analysis by British-based consultancies Ricardo-AEA and Cambridge Econometrics published on Monday studied various scenarios and found improved vehicle technology could deliver between €58 billion and €83 billion a year in fuel savings by 2030 across the European Union.