Renault's relationship with works team Red Bull continues to crumble.
Cyril Abiteboul, chief of the French carmaker's F1 engine programme, on Tuesday accused Red Bull technical boss Adrian Newey of lying.
He admits that Renault is struggling at the start of 2015, insisting: "I am not looking for excuses".
But he said it is wrong of Newey to say the only problem with his 2015 package is the 'power unit' supplied by Renault.
"In addition to our problems," Abiteboul told France's Auto Hebdo magazine, "Red Bull has some chassis problems that do not help the situation.
"In particular, the lack of rear stability. These two problems combine to ensure the car is difficult to drive."
When asked if it is therefore difficult to hear Newey say the only problem with the RB11 is the engine, Abiteboul admitted: "Yes. It's hard to have a partner who lies.
"Adrian is a charming gentleman and an outstanding engineer," he added, "but he has spent his life criticising his engine manufacturers. And he's too old to change."