Red Bull are coming under increasing pressure to resolve their investigation into F1 boss Christian Horner, with Ford criticising their future engine partner over a “lack of transparency.”

Red Bull team principal Horner, 50, strongly denies allegations of “inappropriate behaviour” from a female colleague, which were first publicised on 5 February. He is under investigation by Red Bull Racing’s parent company, Red Bull GmbH.

Horner has repeated those rebukes during public appearances at Red Bull’s car launch and pre-season testing in the three weeks since, though no formal development on the probe has been announced. An outcome is expected in the next few days.

And Ford Motor Co., who will partner with Red Bull in 2026 to form Red Bull-Ford Powertrains, are unhappy with the nature of the investigation, as well as Red Bull’s lack of response to a request for further information on 14 February.

In a new letter, seen by AP, Ford CEO Jim Farley says the American automotive giant are “increasingly frustrated by the lack of resolution or clear indication from you about when you anticipate a fair and just resolution of this matter.”

He adds: “As we have indicated previously, without satisfactory response, Ford’s values are non-negotiable.

“It is imperative that our racing partners share and demonstrate a genuine commitment to those same values. My team and I are available at any time to discuss this matter. We remain insistent on, and hopeful for, a resolution we can all stand behind.

“We are likewise frustrated by the lack of full transparency surrounding this matter with us, your corporate partners, and look forward to receiving a complete account of all findings.”

Farley finishes the letter by calling for Red Bull to show “prompt and serious attention” to the investigation.

F1 also called for a speedy resolution to the investigation last week, while Mercedes boss Toto Wolff insists the probe is a “concern for all” in F1.

Horner insisted at pre-season testing in Bahrain on Thursday that he wants the outcome to the probe “as soon as possible”, with the first race of the 2024 Formula 1 season on Saturday.

Asked why he has not moved aside as team principal and chief executive of Red Bull Racing with the investigation under way, Horner replied: “As you are well aware there is a process going on which I form part of, and as I form part of that process, I am afraid I cannot comment on it.”

Now, the lawyer in charge of the probe will reportedly hand in his findings to the board of Red Bull GmbH – the team’s parent company – in the next few days.

The Red Bull F1 chief, 50, is married to Spice Girl member Geri Horner and is the longest-serving team principal on the F1 grid, having been at the helm since the team’s inception in 2005.

While unable to comment on the nature of the investigation at Red Bull’s launch event last week, Horner appeared composed and undeterred by the allegations, continually insisting it was “business as normal” at the world-championship winning team.

“The support from within the business, partners within the industry, has been overwhelming,” Horner told the British press on 15 February.

Horner, who back in 2021 signed a contract extension at Red Bull until 2026, was made a CBE in the New Year’s Honours list for services to motorsport, having previously received an OBE in 2013.

He has been in charge of Red Bull for six constructors’ championship victories and seven drivers’ championship triumphs – three for Max Verstappen and four for Sebastian Vettel.

Red Bull won 21 out of 22 races in 2023 in their most successful year so far in Formula 1, with Verstappen storming to his third-straight championship.

Yet the investigation has cast a dark cloud over the team’s preparations for the 2024 season, where they and Verstappen will be favourites for title glory once again after the RB20 impressed in testing last week.