The luxury segment is having a tough time these days. Despite resilient sales volumes this year, the brands' residual values are falling.

And Tesla isn't helping matters.

According to data from industry sources, a growing abundance of used luxury-class vehicles is eroding values for their 3-year-old vehicles. The supply of used premium vehicles is up 20 percent since 2006, compared with a 2 percent increase in the overall used-vehicle market, according to J.D. Power.

Robust demand for Tesla's performance electric cars, particularly the Model 3, is accelerating the slide in German luxury used-car prices, a new report by finance company Capital One says. That could pressure profitability at BMW and Mercedes-Benz just as they embark on pricey electrification plans.

Residual values for the German luxe leaders have fallen 6 percentage points since 2015, compared with a 4-point decline in residuals for the overall luxury sector, according to Edmunds.

The market is "becoming flooded" with more affordable cars from Mercedes, Audi, BMW and others — without a corresponding increase in demand, Capital One said.

"It's strong enough to cause prices to plummet, because the market has an excess supply of used luxury cars," it said.And Tesla is complicating the situation by chipping away at the traditional luxury customer base. Tesla's $40,000 midsize electric sedan was the most considered model among luxury shoppers in the second quarter, according to the KBB Brand Watch Survey. And much of that interest came from BMW and Mercedes Benz customers. Tesla receives European vehicles as trade-ins 22.2 percent of the time, more than double the industry average of 10.9 percent, according to Capital One.

"We're calling this The Tesla Effect," the financial institution noted. "It's strong enough to cause prices to plummet, because the market has an excess supply of used luxury cars."

Tesla is a growing brand that is directly targeting the heart of the German luxury market — compact sedans and crossovers, said Jessica Caldwell, executive director of industry analysis at Edmunds.