German luxury car brand Audi AG posted a 22% drop in global sales for the month of September after its European sales suffered from the switch to the new vehicle emissions test in the continent.

Audi sold a total of 139,150 vehicles globally, down from 178,414 cars in the same month last year, according to an Oct. 5 release.

The Volkswagen AG-owned carmaker posted a sharp jump in worldwide sales in August, driven mainly by a 21.5% boost in sales in Europe ahead of the new European WLTP, or Worldwide Harmonized Light Vehicle Test Procedure, switch on Sept. 1.

Most recently, Audi said supply-side factors caused sales in Europe to fall 55.5% year over year to 37,200 units. Audi's sales in its home market of Germany slumped 69.4% to 6,866 units as car manufacturers in the country are now only allowed to register 10% of their prior-year volume of cars that were type-approved according to the previous New European Driving Cycle, or NEDC, test method.

According to the European Automobile Manufacturers' Association, the WLTP test uses real-driving data to assess a vehicle's carbon emissions and fuel consumption values.

"We expected challenging months and are responding actively to the situation," Bram Schot, interim chairman of Audi's board management and board member for sales and marketing, said in a statement.

Four other key European markets also saw sales decrease during the month.

The U.K. saw a year-over-year decline of 53.6%, while Spain posted a 59.4% drop in sales in September. France also saw a decrease of 52.6% year over year, and Italy's sales for the month fell 52%.

Meanwhile, Audi recorded a 24.6% increase in sales in Mexico to 1,237 units, while U.S. sales climbed 0.2% to 19,350, driven by sales of Q5 and A4 models. Sales in mainland China and Hong Kong grew 12.5% to 65,767 units in September.

On a year-to-date basis, the carmaker sold a total of 1,407,700 units from January to September, a 2% increase from the year-ago period.