Paddy Lowe will not return to Williams and has left his role as the chief technical officer of the Formula One team.
Lowe had been serving a leave of absence since March for "personal reasons" with Williams missing the first two days of pre-season testing due to delays with their car.
Williams' season has scarcely improved with the team yet to earn a point, having ended last season on just seven.
"After a period of careful reflection I have reached the decision that I will not return to work at Williams," Lowe said in quotes published on the official F1 website.
"I wish all my previous colleagues the very best to meet the challenges ahead, which I am sure they will do. I would especially like to thank the Williams fans who are so supportive."
Williams' deputy team principal Claire Williams added: "We understand and respect the decision Paddy has reached and wish him well for the future."