Zurich - Fiat Chrysler Chief Executive Sergio Marchionne was being treated in a Zurich hospital on Sunday with a serious illness after suffering complications following shoulder surgery.
A Fiat Chrysler spokesman confirmed Marchionne was in Zurich's University Hospital, one of the country's largest medical centres.
The spokesman did not give the 66-year-old Italian-Canadian executive's condition.
On Saturday Fiat Chrysler announced that Jeep division boss Mike Manley would take over immediately from Marchionne, who had been due to step down next April.
SGS, the Swiss logistics services company, also announced on Sunday that it had named a new acting chairman to take over for Marchionne, since his illness prevented him from fulfilling the role's obligations.
Marchionne was credited with rescuing Fiat and Chrysler from bankruptcy after taking the Italian carmaker's wheel in 2004.
On Saturday he was also replaced as chairman and CEO of Ferrari and chairman of tractor maker CNH Industrial - both spun off from FCA in recent years.
In additional management changes linked to Marchionne's illness, Ferrari named FCA Chairman and Agnelli family scion John Elkann as new chairman, while board member Louis Camilleri becomes chief executive.