Christian Horner has cast fresh doubt over Red Bull's Formula One future beyond this year as their relationship with engine supplier Renault heads for the scrapheap.
Red Bull, powered by the French manufacturer, won four consecutive driver and constructors' championships between 2010 and 2013, but they have failed to win a race in over a year and have finished on the podium on just two occasions this term.
The British-based team have a contract with Renault for next year, but it appears unlikely their fractured partnership will last beyond this season.
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Mercedes will not supply Red Bull with an engine in 2016, which appears to leave Ferrari as the team's only viable option.
Red Bull's outspoken Austrian owner Dietrich Mateschitz has already said his team will withdraw from the sport if they fail to find a competitive engine supplier. And speaking ahead of this week's Singapore Grand Prix, his comments were echoed by Horner.
"You can clearly see that there are two competitive power units in Formula One and you do not have to be a rocket scientist to work out which two those are," Horner told Press Association Sport.
"It is quite clear that one of those will not supply us with an engine, so that leaves really only one option and if that option is not available then you have got to question - as Dietrich made clear earlier in the year - he wants to race with a competitive engine or we won't have an engine."
Asked if that meant Red Bull would not be on the grid, Horner replied: "It is a risk, and it is a risk that should not be underestimated. If we have no engine, you cannot push the car.
"There needs to be some sort of grown-up decisions made about the situation that we are in and I am sure things will become clear in a few weeks."
Renault's chairman and chief executive officer Carlos Ghosn said earlier this week it was "renegotiating" its deal with Red Bull which expires at the end of next season, but their partnership appears beyond repair.
Red Bull have been fiercely critical of Renault and are on course to finish only fourth in this season's championship which will see them lose out on an estimated £13million in prize money.
"You have to consider the loss in revenue through dropping down the order, which is quite enormous," added Horner.
"The most important thing for us is to have a competitive power unit and whilst that may have a short-term impact, the longer-term gain far outweighs that.
"We have to see what Renault's clear plan is and we have to have those discussions with Renault. As we sit here we have a contract until the end of 2016."
Speaking on Thursday, Sebastian Vettel appeared to express his disappointment at the way in which Renault has been treated by his former employers.
Horner responded: "We have had nine years with Renault and during those nine years we have probably contributed more than 150million euros to the operation.
"Unfortunately what we have seen within the new regulations is that Renault has dropped behind, and as a customer if you're with any other product and that product is not working, you change it.
"We have been as patient as we can but we cannot afford for there to be further damage through a lack of competitiveness that this power unit currently represents."
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