Porsche has been considering a return to F1 as an engine supplier since attending meetings with series bosses at the Italian Grand Prix in 2017.
It then renewed its interest last year when the worst of the fallout from the emissions scandal had passed.
According to a document released earlier this week by the Morocco Conseil de la Concurrence - the national government requiring applications are subject to mandatory publication once approved - Porsche is poised to buy 50% of Red Bull Technology.
This would pave the way for a powertrain partnership from 2026 onwards and the partial investment in the race team, potentially the first sign of a contingency plan for Red Bull owner Dietrich Mateschitz.
It was widely suggested that the Porsche-Red Bull deal, reckoned to be for 10 years, would be announced at the drink company's home race, the Austrian GP earlier this month.
The Moroccan document, meanwhile, contains a 4 August date for the tie-up to go public.
But team boss Horner reckons a "lengthy process" still lies in wait, following an FIA-induced delay over the exact specification of the 2026 powertrain regulations.
Porsche and Audi's involvement is believed to be contingent on the MGU-H being ditched, a greater reliance on sustainable fuels and a significant enough reset to allow them to be competitive.
Horner said: "There's some major caveats that we need to get past first before things can get anywhere near progression.
"That primarily focuses on what are the final technical, sporting and financial regulations for the power unit going to be.
"Are they going to be fair and equitable for the newcomers versus the current incumbents? That is the first piece of the jigsaw that needs to be completed.
"It's something that I know the FIA are working hard on. Hopefully in coming weeks we'll get to see that.
"At that point then, we're able then to try and have a further discussion with the guys at Porsche.
"It's going to be a reasonably lengthy process, I would assume.
"The most fundamental thing is, what are those regulations for 2026, and are they attractive enough for an entity like a Porsche or an Audi to come into Formula 1?"
Horner stressed the need for Porsche to fit the 'Red Bull philosophy', adding that it would be "absolutely fundamental to any discussion of not changing that".
As part of match the team's "DNA", Red Bull is seeking a longer-term commitment from Porsche.
"We're really only at a discussion stage and there's so many caveats based on regulations," said Horner. Red Bull has demonstrated its commitment to Formula 1, its longevity in the sport.
"Anything that we look at is very much with the long-term in mind. We're not looking at a short-term solution.
"Strategically, it would have to fit obviously within the long-term plans that Red Bull have for its commitment in Formula 1."
He also reckoned the new Red Bull Powertrains site, with the 'Rindt' factory built in 55 weeks, is "due to fire up shortly" its first engine.