Japan Airlines Corp. is preparing for an initial public offering that may make it the world's most valuable carrier even as new low-fare airlines threaten to lure away travelers in its home market.

Shareholders of JAL, which exited bankruptcy protection in March, are considering selling as much as ¥1 trillion worth of stock as early as September, according to two sources. That would be enough for the Tokyo-based airline to surpass Air China Ltd. as the world's biggest carrier by market value.

The sale may struggle to reach the highest amount possible because concerns about competition may outweigh the benefits of a turnaround that has enabled JAL to return to profit, said Senri Sasahara at Innovative Advisor Corp. The airline faces new challenges this year from two budget ventures backed by All Nippon Airways Co. and JAL itself is also forming a low-cost carrier with Qantas Airways Ltd.