(Adds Daimler comment)

By Norihiko Shirouzu

BEIJING July 12 (Reuters) - Prices for German luxury cars in China are tumbling as the country's stock market sell-off and worries about broader economic growth chill demand for auto brands that once commanded price premiums from affluent Chinese consumers.

After a fall-off in customer traffic and orders, a dozen BMW stores run by a dealer group with showrooms across China are being forced to provide increasingly steep discounts to entice customers, according to the head of the chain.

He said business had already been weak, due to China's slowing economic growth and a corruption crackdown that has weighed on sales of flashy cars, and his stores had to offer a 5 percent discount to grease sales last year.

Over the last week or two, as a stock market rout pummelled the net worth of potential buyers, many of those stores were forced to offer steeper discounts on cars such as the BMW X6 crossover SUV, according to the dealer group chief.

"The business has been slow for the last 18 months, but lately we have had to discount even more," the dealer operator said, asking for anonymity because he did not want to damage his group's relationship with BMW.

"When people walk into a showroom now, with anything less than 15 percent discount they would not even consider opening their wallets."

BMW said in a statement it understood it was "necessary to support the dealers effectively in a volatile market" and had implemented a number of measures, including reducing shipments to help dealers reduce inventory levels and "various" steps to manage their cash flows.

The company was also offering its dealers help in developing after-sales services and used cars businesses to enhance profitability, as well as launching new models, it said.

Discounts and price adjustments are being implemented not just among premium cars brands, but appear to have been happening broadly across China's passenger car market.

The volume-weighted average "maker suggested retail price" (MSRP) for all passenger cars remains relatively high in China, at around 280,000 yuan ($45,000), according to research firm JATO Dynamics.

But actual prices customers pay when buying cars have fallen steadily since 2012 to slightly less than 170,000 yuan, chiefly because of heavy discounting by dealers, according to JATO.

At Mercedes-Benz stores operated by a dealer group with nearly 200 multiple brand outlets, customer traffic at showrooms has dwindled markedly since mid-June, when the stock market slide that saw indexes plunge by as much as a third began.