Christian Horner says Red Bull is seeking a 'first-class' engine supply to replace Renault as fears grow the team - along with sister outfit Toro Rosso - could exit the sport if it can't secure the deal it is seeking from Ferrari.

With Red Bull's divorce from current supplier Renault all-but-finalised with five races of the 2015 season remaining, the Austrian firm is busily attempting to source a replacement for both itself and Toro Rosso, with Ferrari now its sole remaining option.

However, as Ferrari hesitates on a potential deal over Red Bull's desire to secure a factory-specification supply from the Italian firm, Dietrich Mateschitz has made it clear that a failure to land some sort of parity will see the four-time champions exit the sport.

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It is a view reiterated by Horner, who says talks with Ferrari are ongoing, but will hinge on whether it can get 'what it needs' from the rival manufacturer.

The situation is quite critical because, as we sit here, we don't have an engine," he said. "The important thing for us is to have a first-class engine...

"First of all, we need to conclude our situation with our current supplier, but I think Dietrich has made the situation very clear in terms of what we need from Ferrari. We know that there is an engine available at, say, a Sauber-spec service potentially, but even that has not been promised.

"[If we don't get an engine] then we are in a position where we can't compete and then, for sure, we would have to look at other activities. Milton Keynes is full of talent and we would have to look at where we apply that talent."