Mercedes and Renault have cast doubt on the new engine regulations proposed for Formula 1 in 2021.
Mercedes F1 boss Toto Wolff says he has "strong scepticism" about whether the ideas are the right way forward.
Renault Sport F1 boss Cyril Abiteboul said the proposal, made by governing body the FIA and F1 Group on Tuesday, was "a starting point for discussions".
Both said the problem was that they meant a new engine design that required a major financial investment.
What are the proposed new rules?
The FIA and commercial rights holders Liberty Media have proposed keeping the current engine architecture of a 1.6-litre turbo V6 hybrid but removing one of the two hybrid elements, increasing the power of the other, introducing driver-controlled hybrid deployment and standardising a series of parts.
Their aim is to reduce costs, improve the quality of the racing and the sound created by the engines, and enable independent companies to come into F1 and compete with the car manufacturers as engine suppliers.
Why do Mercedes and Renault object?
Wolff said he was "surprised" that the FIA had published so much detail on the new engine when the manufacturers had been told in a meeting on the same day that the plans were "a proposal of a vision for 2021" that would be subject to further discussion and refinement at F1's various rule-making entities.
He added: "It portrays it in a way of this is how we're going forward and none of the current OEMs (car manufacturers in F1) was particularly impressed."
He said the proposals as published would mean "developing a new engine concept that will trigger immense costs" for the car manufacturers in F1 - Mercedes, Ferrari, Renault and Honda - "just for the sake of having a new concept".
"The new concept needs to tackle the deficit that has been outlined - development costs and noise level - and all that needs to be linked with a global view of F1," Wolff said. "We haven't seen any of that."
Abiteboul concurred, saying that Renault objected to being "presented with a new engine on which we would have to make substantial development and substantial financial commitment without an understanding of the broader picture of what F1 would look like past 2020. And I'm referring to not just engine regulations but chassis regulations as well as the commercial side of F1."
He added that Renault "approved of the targets that have been set in terms of cost, noise and power, and the fact we need to try to make performance continue to converge". But he said: "Those are things that could have been done with the current engine architecture anyway."
Honda and Ferrari, the other engine two manufacturers in F1, have declined to comment.
Would new rules attract new companies?
Abiteboul said that he did not believe the proposed new regulations would enable independent companies that have expressed an interest in F1 to enter the sport and compete with the car manufacturers.
"I don't see how what has been presented would be offering a model for an independent engine manufacturer," he said.
"Maybe it would lower the cost of access for a car maker, but you would still need a substantial amount of marketing dollars to spend into research and development to make any business plan work for the new engine.
"And that is actually our problem, that we need to spend again, just like a new entrant would have to spend.
"But I don't think an Ilmor or a Cosworth will be able to go for it independently without the subsidises of another car company."
Sources say that the other engine companies present in the meeting with the FIA and F1 and on Tuesday admitted this was the case.
Disagreement on technology and cost
One key aspect of the proposed new rules is the dropping of the MGU-H, the part of the hybrid system which recovers energy from the turbo.
This is a crucial part of making modern F1 engines set new standards of efficiency - it provides about 60% of the total hybrid energy of the engine - but it has been criticised for being too expensive and for strangling the sound produced by the cars.
Abiteboul said: "My problem with the removal of the MGU-H is that as soon as you do that it is a new engine.
"It fundamentally changes the way the energy is managed within the engine, the way the turbo is working and so on and so forth. It is a new combustion concept, a new way to manage turbo lag and efficiency, so it is new design of turbo."
Abiteboul said Renault had contributed a technical view to the FIA's work on potentially removing the MGU-H, but added: "That doesn't mean we support it. We actually believe that the MGU-H is a fantastic device in order to have a sustainable power around the lap and during the whole race. So that is the sort of disconnect we may be finding ourselves in right now."
Wolff would not discuss the detail of the proposals but insiders say it is clear why the new engine as detailed would increase costs and lead to a fundamentally new design.
- Increasing the revs used by engines, to improve the noise, would mean developing the engine to be able sustain the extra forces involved
- A more powerful MGU-K - the hybrid part that recovers energy from the rear axle - with driver deployment means a new MGU-K design and software to control it, as well as new forms of energy storage and deployment
- A turbo with dimensional constraints means a new turbo design; the same for the introduction of a standard energy store and fuel
In addition, removal of the MGU-H, which recovers energy from the turbo, and raising the revs would decrease the efficiency of the engine, which would mean cars would need more fuel, which would make them heavier at a time when there are already criticisms that they weigh too much.