Red Bull have revealed why Honda only agreed to a single-year extension of their partnership in 2021.

The Japanese manufacturer signed a two-year deal when they expanded to supply both Red Bull teams from last season, taking them up to the introduction of the new regulations coming for next year.

Initially, Formula 1 owners Liberty Media had planned to overhaul the power units as part of the changes but they were dropped when no new manufacturers were tempted to join the grid, as was hoped.

Despite the relative stability in the engine rules, however, Red Bull boss Christian Horner explained there is still one factor that is driving Honda's caution.

"First of all, it's great news they've committed beyond 2020," he began, talking to Sky Sports. "So we've got a minimum of two years that we're absolutely fixed on.

"They are just waiting to see what are the regulations that are going to come in for either '21 or primarily '22 in regards to homologation or engine freezes to get the costs down.

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"These engines are enormously expensive and [due to] the technology involved, all the manufacturers together want to see some cost reduction in that area.

"Of course, we were talking about the future, the discussion just wasn't about '21. There is a real appetite there to build on what we've achieved so far in this relationship.

"It's a great partnership between Red Bull and Honda, as we've seen this year. The key aspect moving forward is absolute clarity on those rules."

The thought of Honda leaving may surprise some given the rapid progress made and the potential of winning races and even championships with Red Bull.

And technical director Toyoharu Tanabe explained what has been critical in making the big performance gains.

"I believe we have learned what is a priority for qualifying, or how to win the race," he told Motorsport. "I'm asking [the factory in] Sakura on the Japan side to focus on some key points to improve. We learned a lot.

"On the other hand, reliability is not perfect enough this year but our reliability is getting much better since last year. It means we can develop positive items, not a countermeasure test or bench test.

"We spent a lot of time on something that was a clean-up test [before] and now we can put more time to improve our performance. Then we can think about the more detail with each area."