Car sales across Europe dropped 23.5% in September as a new methodology for measuring exhaust pipe emissions tests became mandatory across the EU, forcing some manufacturers of vehicles yet to be certified to delay deliveries.

The "Worldwide Harmonised Light Vehicle Test Procedure," or WLTP, came into force on Sept. 1. The emissions-measurement system was developed by experts from the European Union and the United Nations Economic Commission for Europe. The system will measure fuel consumption and CO2 emissions from cars just like the test it replaces, but the methodology has been changed to make the test more rigorous and better reflect the way cars are driven on the road.

New passenger car registrations, or sales, in the European Union reached 1,091,220 units in September, down from 1,466,243 in September 2017, according to Brussels-based European Automobile Manufacturers Association, or ACEA. The ACEA excludes the EU member country of Malta in its tally.

"This should not come as a surprise, as the introduction of the new WLTP test at the beginning of last month caused an exceptional surge in registrations in August," the ACEA said.

In August, car sales across Europe rose 31.2% year over year to 1,134,288 units.

Researcher LMC Automotive said the transition to the new fuel economy rules also led to the unavailability of some models that had not received approval under the new procedure. LMC Automotive expects sales to pick up toward the end of the year as some of the testing backlogs clear.

Car sales slump in all major EU markets

The major European markets saw demand fall in September.

Germany registered a decline of 30.5% in registrations in September on a year-over-year basis. The U.K. posted a 20.5% decline in sales, while France saw registrations drop 12.8%.

Sales in Italy were down 25.4%, while Spain posted a 17% decline. Car registrations in Romania fell 73.4% to 2,771 units on a year-over-year basis, while registrations dropped 41.8% in Austria.

Among the manufacturers, Volkswagen AG's sales slumped 48%, while Fiat Chrysler Automobiles NV's registrations dropped 31.9%. France's Renault SA recorded a decrease of 27% in registrations.

Sales of Kia Motors Corp. were up 2.9% year over year.

Volkswagen saw a 52.2% slide in its flagship Volkswagen brand, while sales of its Audi brand were down 60.7% year over year. The company's Porsche brand also saw a 68.8% decrease in sales.

European sales at Fiat Chrysler's Fiat brand were down 32.8%, while the company's Alfa Romeo brand recorded a decrease of 60.5% in registrations.

Volkswagen's market share slips

Volkswagen's market share in Europe's new passenger-car market narrowed to 15.8% in September, as the market share of Peugeot maker PSA Group increased to 18.2% on a year-over-year basis.

Fiat Chrysler's market share dropped to 5.5% from 6.2%, as rival Renault also saw its market share shrink to 8.7%.

By contrast, Ford Motor Co., BMW Group and Daimler AG all saw their market share expand.