Discussions over Formula 1's 2021 engine regulations are in limbo after the current manufacturers questioned the package that has been championed by series bosses and the FIA.
The basic parameters of the simplified regulations, including dropping the MGU-H, were announced back in October.
Discussions have continued since, and after some initial reservations the manufacturers indicated that they had accepted the loss of the MGU-H. The FIA hoped to produce a final set of regulations at the end of June.
However, at the last minute and following discussions at last week's Strategy Group meeting the manufacturers have kicked back, and suggested that they want to keep the current hardware package after all, including the MGU-H.
The main impetus for their position is the fact that no new entrants have committed to coming into F1 in 2021, which was one of the main justifications for the proposed changes.
Porsche has been involved in all the discussions over shaping the 2021 rules - just as VW was when the current V6 hybrid regs were formulated.
However, it has not yet made a decision on launching an F1 programme, at least in part because of the ongoing emission scandal surrounding the wider VW Group. Aston Martin, which has indicated an interest in an F1 project, is also sitting on the fence.
In addition Red Bull's recently-announced works deal with Honda has changed the landscape, as it clearly puts one of F1's biggest players in a stronger position, while also making Aston's involvement less likely.
"It's still very much out for discussion," said Mercedes boss Toto Wolff. "We had a presentation that would have meant a redesign of the engine.
"All four currently in F1 engaged OEMs would have given their preference, with an understanding that we maybe need a bit more noise, and a discussion around fuel consumption that's important.
"But just for the benefit of redesigning an engine without anybody else entering doesn't make a lot of sense.
"So if somebody would commit to come into F1 in the way that all four of us [Mercedes, Ferrari, Renault, Honda] have committed ourselves, go through the lows and highs, the expenses and investment that it needs, then let's discuss engine regulations.
"But if nobody's inside, it's an academic discussion."
Christian Horner also indicated that the lack of new entrants had become an issue. In addition Honda is keen to keep the MGU-H, and Red Bull is backing that position.
"Ultimately the governing body and the commercial rights holder have got to do what they believe is right for the sport," said Horner.
"It doesn't look like there's anybody new coming in, so really it's down to the FIA and Liberty to decide, what do they want?"
Renault F1 boss Cyril Abiteboul made it clear the French manufacturer does not want to be forced into an expensive new development race, telling Autosport "we should always give priority to stability".