NTT DoCoMo Inc. has shelved indefinitely plans for a March launch of a smartphone featuring a new operating system called Tizen, dealing a blow to one of the platform's key backers-- Samsung Electronics Co.

Japan's biggest mobile carrier said Friday that the smartphone market in Japan isn't growing fast enough for it to support a third mobile operating system, a reference to Apple Inc.'s iOS operating system and Google Inc.'s Android, which powers the majority of Samsung's devices. In explaining its decision, Docomo cited research by IDC Japan that showed the country's overall smartphone market had grown only 2.2% in the April to September period from a year earlier.

"The market is not big enough to support three operating systems at this time," Docomo spokesman So Hiroki said Friday.

A Samsung spokesman wasn't immediately available for comment.

For Samsung, one of Tizen's key backers, Docomo's latest announcement is a setback. The South Korean tech giant runs Android on nearly all of its smartphones, an arrangement that has so far benefited both Samsung and Google. But as Chinese companies introduce increasingly attractive devices that also run Android and Google launches its own Android smartphones, Samsung has been eager to launch a platform of its own where it can control the relationships with app developers, as well as the streams of revenue that app sales generate. Other backers of Tizen include Intel Corp., France's Orange SA and the U.K.'s Vodafone Group PLC.

For Docomo, the implications are less clear. Company executives had eagerly awaited Tizen for years. The company, which held out for years against supplying iPhones in Japan, saw the potential to stop users from fleeing to iPhone-supplying rivals such as SoftBank Corp. But in September, it decided to offer the iPhone 5C and iPhone 5S because it couldn't ignore Japan's best-selling smartphone, executives said. The iPhone held a 37% market share in Japan in the six months ended Sept. 30, according to Tokyo's MM Research Institute. Japan remains Apple's fastest-growing market and generates the company's biggest profit margins.

Mr. Hiroki said the company will continue to work to develop Tizen phones. Roy Sugimura, director of technology planning at Docomo, serves as the chairman of the Tizen Association, which oversees the project. The company also denied that its decision to offer the iPhone in Japan had an impact on its decision to postpone its Tizen phone.

An Apple representative wasn't immediately reachable for comment.

This isn't the first delay for Tizen. The operating system's backers originally said that shipments of Tizen phones would begin in late 2012. More recently, Docomo had made plans for a Tizen phone launch in October last year and after another delay, expectations had been reset. Many industry executives are now expecting some Tizen phones geared for commercial launch to be unveiled at the Mobile World Congress, an annual industry conference, next month.

Write to Mayumi Negishi at mayumi.negishi@wsj.com and Jonathan Cheng at jonathan.cheng@wsj.com

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