Mercedes is not willing to supply engines to Red Bull after Honda leaves Formula 1 at the end of next year.

On the face of it, there are four options for the energy drink-owned outfit: customer Mercedes, Ferrari or Renault deals, or going it alone by buying the Honda engine IP.

"It's something we need to start considering now," said Ferrari's Mattia Binotto when asked if a Maranello-made power unit might be an option.

Dr Helmut Marko said rumours that VW or Porsche might enter the sport to team up with Red Bull is "not realistic".

Renault, however, is an option, even though many consider it unlikely given how the previous relationship ended.

"We know the regulation," boss Cyril Abiteboul said. "When you are a participant to the sport you have to accept the rules.

"But I think we are very far down in the pecking order before they call us again."

But former Red Bull driver Daniel Ricciardo thinks a renewed Red Bull-Renault collaboration could work.

"They didn't get along a few years ago, but there's emotions and then there's business so I don't think it's irreparable. I'm not worried for them," he is quoted by Autosprint.

Toto Wolff, though, issued a firm and clear "no" when asked if Mercedes might be willing to supply customer engines to its closest rival.

"Various reasons, the main one being that we are already supplying four teams including us," he said.

"There is no capacity, but I have no doubt that Helmut (Marko) will have a plan B and probably doesn't need to rely on any of the current power unit suppliers."

Marko, Christian Horner and team owner Dietrich Mateschitz met last week in Salzburg to discuss the situation.

"We are examining all the possible ways we can achieve a competitive engine," Marko told Osterreich newspaper. "That should be clarified by the end of November."

More broadly, Honda's withdrawal has triggered a debate in the Formula 1 paddock about the need to change tack for the future with better engine regulations.

"Maybe one thing we can do is a group of experts to do some advanced research for the next generation of power unit to make sure that it is right in terms of show, in terms of cost, in terms of competitiveness and in terms or marketing platform,," said Abiteboul.

"We should do that sooner rather than later."