Red Bull won the support of Mercedes on Saturday for a call to freeze Formula One's engine rules so they might continue using Honda's power unit after the Japanese have left at the end of 2021.
"The more we look, there really only is one option that works," Red Bull boss Christian Horner told Sky Sports television at the Portuguese Grand Prix.
"And that would be to try and agree something with Honda where we could take on the IP (intellectual property) for the Honda engine, but of course that would have to be dependant on the regulations."
Horner said the plan only made sense for an independent outfit if the astronomical costs could be removed by freezing development.
Formula One's next engine has yet to be finalised but is scheduled for introduction in 2026.
The sport is otherwise due for a major rules overhaul in 2022, having postponed it from 2021 due to the COVID-19 pandemic.
Mercedes, who have won every championship in the V6 turbo hybrid era that started in 2014, have ruled out supplying Red Bull but motorsport head Toto Wolff told reporters he would support a freeze.
"I totally understand where Red Bull is coming from," he said.
"They have the capability of tweaking it (the Honda engine) and maybe optimising it and maybe there are a few things in the pipeline from Honda giving them confidence that there is more performance.
"I think we should be doing everything to give Red Bull that opportunity," said the Austrian.
Without the Honda option, Renault would be obliged to supply Red Bull as the manufacturer supplying fewest teams -- a move neither side would welcome.
Red Bull won both titles for four years in a row with Renault from 2010-13 but fell out with the French manufacturer in the new era.
"It's a big wake-up call for Formula One to have a major manufacturer like Honda walk away from the sport at the end of 2021," said Horner. "That leaves only three engine suppliers, and that's a very precarious place for the sport to be.
"The governing body really need to take control of this."
Asked about a Plan B, Horner said the full focus was Plan A.
"Renault don't really want to supply us," he added. "It's inconvenient to supply a team like Red Bull, we're not a standard customer team. We're not a small team."
Horner said it would be "criminal" to see the Honda engines otherwise left languishing "on a shelf somewhere in a Japanese warehouse."
Honda have won twice this season with Red Bull and sister team AlphaTauri, making the Japanese company the first engine maker to win with two different teams in the V6 era.
They announced this month that they were leaving Formula One to focus on zero-emission technology,