Walter Energy Inc., an unprofitable U.S. coal miner, said there’s “substantial doubt†about its ability to continue as a going concern and raised the possibility of a bankruptcy filing amid low prices and a high level of debt. That warning, made in Walter’s quarterly 10-Q filing on Tuesday, comes 10 days before a deadline for a $62.4 million interest payment due to bondholders. Unless the payment is made, or Walter restructures its debt with creditors, the company will be in default. Walter had more than $3 billion of debt as of March 31, according to the filing. Should it seek bankruptcy protection, it would join U.S. producers Patriot Coal Corp. and James River Coal Co. which have filed since the start of 2012. Walter has struggled after a slump in the price of metallurgical coal following a slowdown in demand in China, the world’s largest producer of steel. Walter scrapped its dividend in January, and last month it agreed to delay the interest payment on the bonds by taking a 30-day “grace period.â€