Red Bull boss Christian Horner doubts outgoing engine supplier Renault will be in a position to compete with Mercedes and Ferrari before 2018.

Red Bull and Renault's partnership, which dominated the sport in the early 2010s, is set to end this year following the breakdown in its relationship since the introduction of V6 turbo engines. Most of the frustration on Red Bull's side came from Renault's uncompetitive power unit, which has continued to struggle for performance and reliability in 2015.

Renault is hoping to return to the grid next season as a manufacturer and is moving towards a takeover of Lotus, while Red Bull is in talks to secure Ferrari power for 2016. Horner says the current state of Renault's power unit prompted the early termination of its contract.

"Since the power unit regulation change, it's a very different world that we're living in," Horner told Sky Sports F1. "There's really only two engines out there that you can compete for grand prix victories with and, unfortunately, Renault have fallen behind that. It looks like it's going to be at least two to three years before they can be in a position to compete again. As a paying customer, we can't afford to wait that long."

Red Bull has threatened to quit Formula One if it is not given an equal engine by Ferrari for 2016, a deal which looks extremely unlikely to happen. F1 boss Bernie Ecclestone is pushing for a compromise in order to save Red Bull and junior team Toro Rosso from dropping off the grid next year.

The quit threats followed several months of publicly criticising Renault, but Horner does not believe the team's conduct this year has dissuaded other suppliers from bailing them out, despite Mercedes opting against an engine deal.

"I doubt it," Horner said. "Perhaps we have been guilty of being honest, whether that is my comments or Adrian Newey's comments or Helmut Marko's or most important of all Dietrich [Mateschitz]'s as he is paying the bill at the end of the day. I don't believe that would have an impact on influencing any other supplier."