Peabody Energy, the largest US coal miner, warned Wednesday that it may be forced to seek bankruptcy protection after missing a key debt payment.

Just two months after the number-two miner Arch Coal fell into bankruptcy, Peabody said that due to operating losses and negative cash flows, it risked falling into default on its loans.

The company said it had invoked a 30-day grace period on debt payments of $71 million due on Tuesday.

While that did not yet constitute a default, it said in a securities filing, with its $6.3 billion debt load at the end of 2015, "there exists substantial doubt whether we will be able to continue as a going concern."

"We may not have sufficient liquidity to sustain operations," the St. Louis, Missouri-based company said, adding: "We may need to voluntarily seek protection under Chapter 11 of the US Bankruptcy Code."

Peabody shares plunged 42.6 percent to $2.30 in opening trade on Wednesday. The shares were trading above $80 a year ago.

Peabody lost $2.04 billion last year on $5.6 billion in revenues, as both coal prices and volumes shipped sank.

The company, like the entire US coal industry, has been hit hard by the plunge in coal prices as energy users from power plants to steel factories turn away to more environmentally friendly, and also very cheap, natural gas and other alternatives.

It said it also faced stagnant economic growth in major international coal importers, and the rising costs of environmental regulations.

Around two dozen US coal companies have sought bankruptcy protection or closed in the past three years due to the fall in coal demand.

Peabody's financial weakness  has been well-known and the company has sought to lighten its current debt burden in talks with lenders.

At least one lender though has been urging the company to enter Chapter 11 bankruptcy for a court-supervised debt restructuring.