Volkswagen warned business was becoming "difficult" after reporting the weakest monthly growth rate so far this year, while Ford blamed falling European sales on heavy rebates offered by rivals.
VW said it delivered 572,200 vehicles in July, up just 2.9 per cent from a year earlier, a steady deceleration from the growth rates of 5.7 per cent in June and 8.6 per cent in May. Earlier in the year the pace was topping out at 20 per cent or more.
"Over the coming months we will continue on our growth path with our model range and will perform better than the competition. However, this will be a challenge, given an operating environment that is again becoming difficult," sales chief Christian Klingler said in a statement.
"Now that (government-subsidised scrapping) incentive programmes have come to an end, the global automotive market is expected to decline in the second half of the year. There will not be a return to the high pre-crisis levels this year."
Nonetheless, Volkswagen's group vehicle sales eclipsed the 4 million mark for the first time ever during the first seven months through July, rising 13.7 per cent thanks mainly to the success of its core VW and Audi brands in China.
"Apart from the Golf family and the Tiguan, demand in China for the very successful Lavida, Passat Lingyu, Jetta and New Bora models was particularly high," Volkswagen said.
Its troubled Spanish brand Seat managed to grow only 2.3 per cent through July, significantly underperforming the 13.4 per cent growth in the global auto market.
Even though sales have rebounded in Spain, Seat's weak brand image means it is being squeezed in its core domestic market between low-end upstarts like Renault's Dacia and GM's Chevrolet and the more aspirational brands like Peugeot and its own sister marquee Volkswagen.
Separately, Ford of Europe reported its share of the western and central European car market dropped by a hefty 70 basis points to 8.3 per cent last month amid a 22 per cent decline in sales.
"High discounts on new cars - as many of our competitors are continuing to offer - can have the negative effect of lowering the residual value of a vehicle and imply a risk to the investment a customer has made when buying a new car," said Ford of Europe sales chief Ingvar Sviggum in a statement.
"I believe such unsustainable heavy discounting only damages brand reputation and further weakens the market," he said.
Reuters