BEIJING, May 14 (Reuters) - China's state-owned planemaker COMAC has preliminary drafts for a new widebody jet called the C939, the South China Morning Post reported, likely a larger version of COMAC's first twin-aisle C929 which is itself still under design.

China, through the Commercial Aircraft Corporation of China (COMAC), is investing heavily in designing and manufacturing homegrown aircraft in an attempt to break into a passenger jet market dominated by Western planemakers Boeing and Airbus .

COMAC has sketched out preliminary designs for the new C939 aircraft but it will be many years before a testable prototype could emerge, according to the SCMP report published late on Monday citing a source familiar with the matter.

No other details about the C939 were given. Comac did not immediately respond to a Reuters request for comment.

COMAC has put two planes into commercial operation: the ARJ21 regional jet which entered service in 2016 and flies in China and with Indonesian carrier TransNusa, and the C919 single-aisle which began flying passengers in May 2023 and is so far only certified within China.

But its long-haul wide-body program has experienced delays. This included Russia in 2023 dropping out of a joint venture to develop the C929.

COMAC has previously said the C929 will have around 280-400 seats and a range of 12,000km (7,500 miles), which is in the same category as Boeing's 787 Dreamliner.

A report in April from the C929's fuselage manufacturer said it aimed to deliver the first fuselage section by September 2027. A second source in the SCMP report said progress on the C929 was "in full swing".

COMAC is expanding production facilities for the C919 which has received more than 1,000 orders - nearly all of which are from Chinese airlines and lessors.

China's aviation authority has said it would promote the C919 internationally this year, and it made its debut international flight at the Singapore Airshow in February.

COMAC is also pursuing European Union Aviation Safety Agency (EASA) certification for the C919 - a benchmark approval that non-Chinese airlines and customers have said they would expect before considering purchases. (Reporting by Bernard Orr and Lisa Barrington; Editing by Jamie Freed and Lincoln Feast.)