Williams performance chief Rob Smedley will leave the Formula 1 team at the end of the 2018 season.
Smedley joined Williams from Ferrari in 2014 as head of performance engineering in a newly-created role as the team sought to bounce back from a tough 2013 campaign.
He helped guide Williams to back-to-back third-place finishes in the constructors' championship, then fifth place in 2016 and 2017.
Smedley said he has not decided on what he will do next, while Williams has no immediate replacement as it is thought to still be evaluating changes behind-the-scenes in response to its tough 2018 season.
He will depart Williams at the end of another disappointing year for the team, which also lost Dirk de Beer (head of aerodynamics) and Ed Wood (chief designer) in May.
"I have thoroughly enjoyed my time at Williams," said Smedley. "The team has been through great change since I joined in 2014 and it has been a pleasure to have played a part in that.
"Williams is a very special team within the F1 community and I'm certain that with all the talent that we have here they will go on and do better things.
"The team will always remain close to my heart. After 20 years in Formula 1, however, I feel it's the right time to reflect on things and evaluate what the next move is.
"I'm thoroughly looking forward to spending more time with my family before deciding on future opportunities."
Smedley's F1 career started 17 years ago with Jordan, before joining Ferrari three years later in 2004.
He did not immediately join long-time Ferrari companion Felipe Massa at Williams at the start of the 2014 and had to wait until just before the third round of the season in Bahrain to start working with the team.
Smedley joined as part of an overhaul of the team's engineering department, initially in charge of the Williams pitwall on race weekends and as the senior trackside engineer under then-chief technical officer Pat Symonds.
He then moved into a broader role looking beyond only race-weekend performance.
Claire Williams, deputy team principal, said: "It has been a pleasure having Rob in our team for the last four years and we will miss him both personally and professionally.
"He agreed to join us at a time when our performance was low, and we are grateful that he saw the potential for us to turn things around at that time.
"During his time with us, he has made a significant contribution to the team through his role, but he's also been a great person to have at Grove, inspiring many with his enthusiasm and passion for Formula 1.
"Rob's been in the sport a long time, so we totally understand and respect his decision to take some well-deserved time out to spend with his wife and boys.
"We wish him all the best and thank him wholeheartedly for the time, passion and fighting spirit he's given to Williams."
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