The European Commission is due to propose amendments to its landmark ban next week -- having come under intense pressure to weaken it from some big car-producing nations, like Germany and Italy.

In the other camp are France, the Nordic countries and Spain who have long called for keeping to the trajectory to shift to electric vehicles in order not to harm firms that have made investments in the transition.

Madrid put its reservations on paper in a letter to commission chief Ursula von der Leyen dated Thursday and seen by AFP Friday.

"Any additional relaxation would risk triggering a significant delay in modernization investments, linked to a temporary slowdown in electric vehicle demand, directly impacting the future competitiveness of the European automobile and auxiliary industry," wrote socialist Prime Minister Pedro Sanchez.

"We therefore reject that combustion vehicles or other technologies without proven viability could continue to be marketed beyond 2035."

European automakers have been clamouring for flexibility in the face of fierce competition from China and a slower-than-expected shift to electric vehicles (EVs).

Spain's missive came as Manfred Weber, the head of the EU parliament's largest group, the EPP, said the 2035 ban on new petrol and diesel car sales would be discarded.

Weber told German tabloid Bild on Thursday after a meeting with EU chief Ursula von der Leyen -- who hails from the same political family -- that carmakers would be required to meet a 90-percent reduction in CO2 emissions from their fleet.

Commission officials have stressed that no final decision has been made.

But sources within the EU body suggested to AFP that the proposal in the works will likely include the 90-percent target and allow for plug-in hybrids and range-extender vehicles to be sold after 2035.

This has environmentalists worried, with a recent report indicating that plug-ins pollute almost as much as petrol cars. But it would meet some of the demands of Berlin, Rome, and others.

"There is a clear demand for more flexibility on the CO2 targets," commission spokeswoman Paula Pinho told a press conference Friday, saying Brussels was "aiming for balance".

Road transport accounts for about 20 percent of total planet-warming emissions in Europe, and 61 percent of those come from cars' exhaust pipes, according to the EU.