Jaguar Land Rover sales fell in September, the first monthly fall since July 2011 and a blow for Indian parent Tata Motors. The luxury vehicles make up 90% of the group's profit.
JLR's sales growth over the past year, helped by the new Evoque model and success in emerging markets such as China, had been offsetting sluggish growth in Tata's domestic business and driven its stock price up 50% over the past 12 months.
The British brands sales fell 4% in September, year-on-year, with 26,461 vehicles sold. It had previously reported August sales up 13%, down from annual increases of about 40% in the preceding three months.
"These numbers are not good," said Jinesh Gandhi, automotive analyst at Motilal Oswal Securities in Mumbai. "This is below consensus expectations and so the stock is likely to be affected negatively."
JLR, which Tata bought for $2.3 billion in 2008, has helped offset slowing sales of Tata's own branded cars and trucks in India where cooling economic growth and high interest rates have hit demand.
It accounted for about 70% of Tata Motor's consolidated revenue in the April-June quarter, and around 90% of profit.
Tata Motors' overall global vehicle sales were down 4% in September year-on year to 103,656 vehicles, also the first monthly fall since July 2011.