Renault let its own Formula 1 staff down - Red Bull boss Horner

Horner has often questioned how much support Renault has provided, and those ongoing tensions created the momentum for Red Bull to switch to Honda in 2019.

"Credit where credit is due - in Mexico we were provided with an engine that was capable of winning, as we were in Austria and China," Horner told Autosport.

"We were lucky in Monaco, even without an MGU-K. Any other circuit and we would have been stuffed.

"I have total admiration for the Renault guys in the garage that are working their socks off, week in, week out.

"But I think they've ultimately been let down by the main house's lack of commitment to development and reliability."

At times a lack of availability of new parts caused frustration for Red Bull.

"Too often you see parts being taken from one engine to go onto another," Horner added.

"That's been too much of a theme over the whole hybrid era, and that must compromise dyno time, that must compromise endurance running, and so on."

Red Bull was the only one of the three Renault-powered teams to switch to its 'spec C' upgraded engine in the latter part of the season - although it continued to alternate between it and the previous B spec.

Horner said that was an occasion when Red Bull using Exxon/Mobile fuel rather than the BP/Castrol used by Renault and McLaren - a strategy Renault had previously criticised - proved advantageous.

"Exxon have done a very good job for us," he said.

"Their difficulty and frustration has also been that as a customer we get very limited dyno time.

"But they were able to come up with a fuel that made the C-spec work, and we've managed to get that in the car and run that successfully, when the works team haven't been able to do that.

"So that's also been encouraging."

Renault F1 boss Cyril Abiteboul admitted he had mixed emotions about the split with Red Bull.

"It's a bit strange, because 12 years is a big chunk of Red Bull's and Renault's history," he told Autosport.

"And it's a ling time also for the individuals involved, the technical guys, the mechanics on both sides, the engineers.

"We've had some difficult moments, and very good moments also. But I think it will just allow us to focus.

"On many occasions Red Bull diluted our focus, in particular with regard to communication.

"We'll be able to control our communications much better, and they will be be able to develop their relationship with Honda. It's good for both parties."