Honda announced its F1 exit in 2021 but agreed a deal to continue to supply its power unit for chassis partner Red Bull until the end of 2025 and the demise of the current engine regulations.
But the Japanese manufacturer will return to the series in 2026 as the works partner for Aston Martin, with Red Bull having already created its own in-house Powertrains division in the meantime.
Max Verstappen, who then delivered Honda the 2021 and 2022 drivers’ championships and helped secure last season’s constructors’ crown, described it as "a shame" to see Honda partner Aston.
Horner added that if Red Bull had known Honda was keen to return, it never would have embarked on its own engine programme even if it had admittedly “outgrown” being a "customer".
Asked by Autosport for his reaction, Horner said: “Well it’s certainly an expensive decision!
“For the prospects of Red Bull, we’ve outgrown being a customer.
“For us to have the power unit on-site, integrated fully with the chassis and the synergies that creates with engine and chassis engineers next to each other, for the long-term, the advantages are significant.
“We wouldn’t have made that jump had it not been for Honda’s withdrawal."
The creation of Red Bull Powertrains has provided more jobs in Milton Keynes to mitigate the restrictions imposed by the cost cap implemented in 2021.
“In many respects, we should be grateful for them giving us that push to create our own engine facility," he continued.
“The jobs that it’s created and provided and of course, the [2026 commercial-led] partnership we have with Ford is particularly exciting for the future and the commitment from Red Bull and the shareholders to the project.
“Would we have made the same decision knowing what Honda’s decision is today? Absolutely not.
“But we made it and we’re committed to it and as the way we’ve gone has evolved, the more benefit that we see to the group long term.”
Red Bull and Honda did re-enter talks in late 2022 over a possible collaboration for the 2026 engine rules, which ditch the complex and expensive Motor Generator Unit – Heat (MGU-H).
Their discussions focused primarily on the hybrid components of the engine, with Honda having originally left F1 to dedicate resources and finance to electrifying its road car line-up.
However, Horner cited “too many compromises probably from both sides” for the breakdown in the negotiations, before Red Bull used its New York City launch of the RB19 to publicly reveal the Ford deal.
