Miner Anglo American Plc's debt was downgraded further into "junk" territory by Moody's Investor Service, which cited a deterioration in commodities market conditions and doubts over how long it would take the company to reduce debt levels.
Moody's downgraded the company to (P)Ba3 from (P)Baa3, and said the outlook on the ratings was negative.
Moody's said it does not expect Anglo American to generate enough operating cash flows to deliver substantial organic debt reduction in the next two years.
"Pending further announcements by the company, the rating agency believes that divestments of non-core assets would be difficult to execute in the current environment, particularly at valuations to allow deleveraging from the current level," Moody's said in a statement.
It added that the negative outlook reflected uncertainty that Anglo would be able to execute its restructuring.
The commodities slump has sent share prices of mining companies tumbling, with Anglo falling by three-quarters in 2015 and the FTSE Mining Index nearly halving.
Peer Glencore Plc, which saw its share price fall 70 percent in 2015, was downgraded by Moody's to one notch above junk in December.
Anglo American announced in December that it would sell three-fifths of its assets and cut tens of thousands of jobs to cope with the slump.
Anglo is expected to report full-year results on Tuesday. The company's stock closed up 5 percent at 393.05 pence on Monday.