The industry had agreed this year not to release the traditional mid- month figures, but sources said sales in the first 20 days of the month totalled 328,000, a rise of less than 3 per cent on the 319,000 cars sold in the same period last year.
The latest data suggest sales are now unlikely to break through the 500,000 barrier for the first time since 1989, as the dealers' association had predicted.
Figures were more promising in the first 10 days of August, showing an underlying increase of around 5 per cent after adjusting for differences in the number of working days. They were also in line with the industry forecasts.
One of the worst-hit companies this August has been Rover Cars, where the drop of 16 per cent suggests the British manufacturer's steady decline in UK sales is continuing unabated. Between January and July, Rover had just 9.6 per cent of the British market, down from 10.9 per cent during the same period last year.
A spokesman for Rover declined to comment on the figures. "We have a policy this year along with the rest of the industry that we won't discuss figures until the end of the month. So much can happen in the last few days of August." he said.
However, Ford's August has also been disappointing, with overall sales down 10 per cent in the first 20 days and sales of the Mondeo slashed by 28 per cent. Dealers said Ford was offering outlets an additional pounds 750 profit margin on the Mondeo to use as a bargaining tool with potential customers.
Vauxhall's sales so far this month are down 6 per cent, with sluggish demand for the medium-sized Astra and, according to sources, lower than expected interest in the new Vectra, a replacement for the Cavalier.
One dealer, Richard Smith from Cleckheaton Holdings in West Yorkshire, which sells Rover, Ford and Vauxhall, said: "We are definitely disappointed. Things haven't taken off as much as we'd thought. After the first few days interest trickled off quicker than expected."
Manufacturers are understood to be stepping up the number of "pre-registrations," cars which are sold direct to dealers to register in the figures, even though they often remain unsold on garage forecourts for several weeks.
"All the indicators from within the trade suggest this is happening," said Alan Pulham, from the National Franchised Dealers Association. "My impression is that the more conventional dealers are having a good August, but the volume ones more used to fleet sales are having problems."
Volkswagen reported the biggest increase in sales in the first 20 days of the month, with a jump of 43 per cent. The new Polo "super-mini" model accounted for most of the increase, as VW stepped up imports to try to cut waiting lists at some dealers.
Peugeot has also achieved a substantial sales rise on the back of its new 406 model, with dealers also said to be reporting supply shortages.
The managing director of Corby Motor Group, Martin Austin, who sells Peugeots and Renaults, explained: "The increase is partly accounted for by the launch of new models with lots of television advertising, but also action taken by the manufacturers to cut dealer profit margins. It means retail prices are more attactive to customers this year."
Meanwhile, official figures released yesterday show UK manufacturers are increasingly gearing production towards export markets, despite sluggish demand in continental Europe.
In the three months to the end of July, production allocated for export surged by 11.5 per cent, while production for the home market rose by just 0.6 per cent.
Total UK car production rose by 6 per cent on a seasonally adjusted basis, the Office for National Statistics said. Compared with the same three-month period a year ago sales are up by 8 per cent.
August car sales
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