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The group said the decline was due to low demand for commercial vehicles throughout Europe.
A total of 1,282,248 registrations was recorded in the first nine months of the year.
All the major markets, ACEA added, recorded negative figures over the period. Registrations fell by 3.1% in the UK, 4.2% in Germany, 7.7% in France, 25.7% in Spain and 34.6% in Italy.
Year-on-year, the figures for September were mostly even poorer, with 150,910 new commercial vehicles registered in the EU, a decrease of 13.7% year-on-year.
Figures from Greece suffered the most, with commercial vehicle registration down by 51.2% in September year-on-year. Spain saw a reduction of 32.7%, and registrations were down in Italy by 28.2%, in Germany by 16.6% and in France by 13.4%.
Only in the UK, ACEA noted, did new vehicle registrations increase in September, rising by 0.7%.