Market intelligence company Jato says the growth of mini and midi-MPVs is responsible for the fall, along with model replacement cycles.
Sales in the MPV segments combined rose by almost 30% during September compared to the same month the previous year, according to the research.
Jato added: 'Superminis also seem to be growing at the expense of lower-medium models, with a strong September giving a year-to-date increase of more than 8%. The traditional segments continue to fragment.
'The popularity of diesel-powered vehicles expanded further to a 43% market share year-to-date against 40% for the same period last year. The Peugeot 206 was the top-selling car overall for September, with 53,136 units.'
It found that the highest percentage growth was achieved in the luxury segment, which was up more than 55% year-on-year, led by the Mercedes-Benz S-class and BMW 7-series.
For the upper-medium segment, Jato says the decline in September softened slightly to 5% over September 2002, and 10% down year-to-date.
It added that the Vauxhall/Opel Vectra, Toyota Avensis and Mazda6 were bucking the trend, chasing the Volkswagen Passat and Ford Mondeo, which retain the two top slots.
Total passenger car sales in western Europe were down 1.53%, compared to the same period last year, Jato said.