McLaren's former chief engineer Pat Fry is joining Formula One rivals Ferrari as assistant technical director from next month, the Italian team said in a statement.
The 46-year-old, who spent 17 years with British-based McLaren and would have played an important role in designing next year's car, will report directly to technical director Aldo Costa.
McLaren's world champions Lewis Hamilton and Jenson Button are currently first and second in the drivers' championship ahead of Sunday's European Grand Prix in Valencia. The team also lead Red Bull in the constructors' standings.
Ferrari, the sport's most successful team, who recently celebrated their 800th race, are third overall.
"Whenever somebody of Pat's calibre leaves the team it is sad, and we like Pat very much and he is a fine engineer, but people have their lives to lead and want to do other things," McLaren managing director Jonathan Neale told reporters last month.
"It doesn't give me any concern for the performance of the team or what we do for next year's car at all because we are still a team that has strength in depth and we are committed to maintaining that."
Fry had worked for McLaren since 1993, after six years with Benetton and progressed from being race engineer for Mika Hakkinen and David Coulthard to head up the team's engineering operation.