LOS ANGELES -- Mazda has postponed the U.S. launch of its diesel-powered Mazda6 mid-sized sedan until April because of delays in emissions testing, the company said Thursday.

Instead of hitting showrooms before year end as planned, Mazda6 sedans powered by the automaker's Skyactiv-D 2.2-liter turbodiesel will arrive in late April, Robert Davis, senior vice president of U.S. operations for Mazda North American Operations, disclosed to Mazda employees today in a letter obtained by Automotive News.

Mazda later confirmed in a brief statement that the diesel launch would be delayed until late-spring 2014, "to accommodate final emissions testing and certification."

The delay is a speed bump in the company's plan to become the only Japanese automaker to sell a diesel-powered passenger car in the United States. While most other automakers have launched hybrids, plug-in hybrids and electric vehicles to meet tougher fuel economy standards, Mazda charted its own course by making diesel-powered vehicles a key part of its fuel economy strategy.

In the letter, Davis said final emissions certification testing on the powertrain is taking longer than the company expected.

"I know we had discussed it being in showrooms before the end of the year, and everyone involved in the program is disappointed it will not be, but final certification testing -- the results of which are looking encouraging -- is taking longer than we had initially expected," Davis wrote.

A Mazda source close to the situation says that in lieu of the Mazda6 diesel, U.S. dealers will receive about 4,000 additional units of the Mazda6 sedan with regular gasoline engines in the first quarter next year than initially planned. The car is in short supply after sales more than doubled compared to the prior year in June, July and August.