Red Bull owner Dietrich Mateschitz has confirmed his team will not be supplied with Renault engines next season.
Renault, who powered Red Bull to four consecutive driver and constructor titles between 2010 and 2013, have a contract with the British-based team which expires next year.
But both parties have been keen to end their fractious relationship and Mateschitz, speaking ahead of the Singapore Grand Prix, has confirmed the split.
"The separation from our engine partner at the end of the season is by mutual agreement," he told Austrian newspaper Salzburger Nachrichten.
"There was no sense in working together any more. You couldn't put the teams through that any longer."
Red Bull have been fiercely critical of Renault this season as their slide down the field continues. They have not won a race in over a year, have featured on the podium just twice this term, and are set to finish only fourth in the constructors' standings.
Volkswagen are said to be holding talks with Red Bull in a bid to take over the team from 2018.
But their more pressing concern is finding an engine supplier for next season. Ferrari are the frontrunners after Mercedes pulled out of negotiations.
"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 said on Thursday.
"It is quite clear that one of those will not supply us 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.
"If we have no engine, you cannot push the car. It is a risk and it is a risk that should not be underestimated."
Source: PA
