The prospect of Red Bull Racing staying in Formula 1 appears to have been dealt a potentially critical blow after Ferrari reportedly refused to offer engines for the rival team.

With Red Bull's options for an engine supply seemingly hinging on Ferrari after divorcing from Renault and being rejected by arch rivals Mercedes, the Italian firm has been in talks with the former title winners to negotiate a deal.

However, with Red Bull seeking factory-level parity and Ferrari only seemingly willing to supply engines similar to those being offered to customers Haas and Sauber through fear of arming a competitive rival, Auto Motor Und Sport is reporting an agreement has subsequently not been reached.

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To make matters more intriguing, the publication is reporting that Ferrari is still keen to supply the sister Toro Rosso team, though it remains to be seen whether it will be allowed to take up the offer - possibly without Red Bull involvement - or whether the Faenza-based sister team will also withdraw.

Speaking to Sky Sports in the wake of the new revelation, Christian Horner says 'There are a lot of negotiations going on - nothing is fixed.'

Red Bull owner Dietrich Mateschitz has always maintained that the team would exit F1 if it cannot secure a competitive supply of engines, a scenario premeditated by a public spat with Renault over the competitiveness and reliability of its power units.

With Renault set to go it alone through the purchase of the Lotus team, Red Bull is understood to have agreed a 'divorce' with the French firm a year earlier than originally planned, though it isn't confirmed if this has been finalised. With this in mind, there are suggestion Red Bull could yet still retain a Renault supply as a result, though Horner has as recently as this week claimed the manufacturer is 'two or three years' behind development with its engine.

Ahead of the Russian Grand Prix, Bernie Ecclestone says his influence over Ferrari to offer engines is fairly minimal, not least because it is already tied to supplying Haas and Sauber next year.

The only other potential engine supplier is Honda, but with its power unit still struggling for pace and reliability, it will remain exclusive to McLaren in 2016.