Red Bull form continues to anger owner Dietrich Mateschitz
Red Bull owner Dietrich Mateschitz claims his team's uncompetitive form of the past two seasons is draining his "will and motivation" to remain in F1.
The Austrian has repeatedly threatened to quit the sport if Red Bull cannot get hold of a competitive engine.
"What else has to happen that we will lose our motivation completely?" he told Red Bull's Speed Week magazine.
Red Bull have collected just 54 points from the first seven races after dominating the sport from 2010 to 2013.
Sebastian Vettel won four world titles with the team before switching to Ferrari at the end of 2014.
Mateschitz is particularly upset with engine partner Renault for failing to close the gap to world champions Mercedes.
"They take from us not only time and money, but also the will and motivation," said the businessman, who founded the Red Bull energy drink company and has a net worth of $10.7bn (£7.6bn).
"There is no driver and no chassis which is able to compensate for this lack of horsepower."
Mercedes top the constructors' standings after winning six races in 2015.
Red Bull have a contract with Renault until the end of 2016 and are contracted to F1 until 2020.
But Mateschitz said they could still quit, warning: "You can't force someone to stay when he wants to leave."
He also criticised what he thinks are overly restrictive aerodynamic rules that prevent star designer Adrian Newey "from using his full talent".