BERLIN—BMW AG has lost the core development team of its i3 and i8 electric vehicle line to Future Mobility Corp, a Chinese startup backed by Tencent Holdings, as the German premium brand struggles to come up with a convincing answer to competitor Tesla Motors Inc.

Carsten Breitfeld, a 20-year BMW veteran who developed the company’s i8 plug-in hybrid sports car, left the Munich-based car maker last month to become chief executive of the Chinese electric car company. Now, three key executives from the “BMW i” electric car group are following him.

Dirk Abendroth, who developed electric powertrains for the i-series, Benoit Jacob, who was head of design at BMW i, and Henrik Wenders, head of BMW i product management, will take up positions at Future Mobility, according to two people familiar with the situation.

BMW declined to comment on whether the executives had departed.

The defections come as BMW and other premium brand manufacturers are struggling with weak sales of their electric vehicles and a growing threat from new rivals in the nascent electric car market.