The Cowley plant is where BMW manufactures the Mini
UK sales of the Oxfordshire-built Mini have fallen by almost one fifth on the same time last year.
In September, 7,116 of the Cowley-built cars were sold compared with 8,764 in the same month in 2007, which represents a fall of 18.8%.
It comes after the Society of Motor Manufacturers and Traders (SMMT) announced that new UK car registrations fell 21% in September.
But Mini, which is owned by BMW, said it had seen an increase in US sales.
September was the fifth consecutive month that the SMMT reported falls in new car sales.
September is usually second only to March - the other month in which car registration letters change - as the most popular month for buying a new car.
So far this year, 34,816 Minis have been sold compared with 36,969 in the first nine months of 2007.
BMW spokeswoman Angela Stangroom, said the company was keeping a close eye on the market situation.
"We expect to build the same amount of cars this year at Cowley as last year, 237,000," she said.
"We have not cut any shifts."
Earlier this year, hundreds of staff at the Mini factory were laid off over a weekend due to Spanish fuel protests.