Red Bull Formula 1 team principal Christian Horner says the FIA would have to reconsider the independent engine proposal if a way forward with the current rules cannot be found.
The idea was put on the backburner when the four manufacturers - Mercedes, Ferrari, Renault and Honda - agreed to a reduction in the cost of customer deals and provided assurances the whole grid will be supplied.
The FIA said last Tuesday that "significant" progress has been made towards finding a way forward with the cost, performance, noise and supply of power units.
But a definitive set of regulations has yet to be finalised, with further work required and a new deadline set at April 30.
When asked what would happen if Mercedes and Ferrari were not in favour of changes and an agreement could not be found, Horner told Autosport: "That's where the FIA has to reconsider the independent engine again.
"It's unlikely but not impossible."
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Having failed to secure an engine from Mercedes, Ferrari or Honda, Red Bull has stuck with Renault for one more season.
Horner says Red Bull has options for next year but it is limited given no new manufacturers are expected to join.
The Volkswagen Group, which has been rumoured to be considering an F1 entry and Red Bull tie-up last year, has repeatedly reasserted its disinterest in the championship this week.
"Our options are relatively limited and fairly obvious," said Horner. "We will see how things pan out.
"There are no new manufacturers coming in for 2017. It's certainly too late for [them to come in in] 2017."
If Red Bull is forced to remain a customer beyond this season, Horner remains unperturbed.
"The regulations say that equipment has to be the same for a works team and a customer team," he said.
"We won a championship as a customer when Renault still had a works team in 2010."
Horner said Red Bull running its own engine programme was out of the question.
"It's not Red Bull's core business to be an engine manufacturer so it doesn't make sense," he said.