Gerhard Berger says he decided to sell his stake in Scuderia Toro Rosso because he feels the team are set to lose ground under the 2009 rules.
Toro Rosso announced yesterday that Red Bull boss Dietrich Mateschitz had bought back Berger's 50 per cent stake of the team, giving the drinks company total control of STR again.
The team had enjoyed a spectacular 2008 season - taking sixth in the constructors' championship (ahead of the senior Red Bull Racing team) and winning the Italian Grand Prix.
But Berger thinks it will be hard for the small squad to maintain that level of progress after the major regulation changes this winter.
"The new rules leave no room for improvement for a small team as STR," Berger told autosport.com.
"Also, Dietrich's interest will focus on Red Bull Racing, therefore the support for STR will become smaller and prevent the team from improving further from sixth.
"I don't want to decline, that is not in my nature."
But Berger emphasised that he was leaving Toro Rosso on good terms.
"Yes, I will be out of STR, but the split with STR and Dietrich is completely amicable and there was no rift between us."
He also denied speculation that his departure had been prompted by a disagreement over Toro Rosso's 2009 driver choice.
"It has nothing to do with that," Berger insisted.
Autosport.com understands that Toro Rosso are leaning towards Sebastien Buemi and Takuma Sato as their line-up for next season.