The popular US cash-for-clunkers programme may be drawing money from other consumer purchases and could also undermine future car sales, US economists have warned.

Motor vehicle and parts sales, which are down 8 per cent on the year, jumped 2.4 per cent from June, according to retail data from the US commerce department yesterday, but other retail sales fell 0.6 per cent in July, compared with expectations of a 0.1 per cent increase.