Moody's Investors Service on Tuesday cut its rating on General Motors Corp. (GM) deeper into junk territory, citing increased uncertainty about the automaker's ability to achieve competitive wage, benefit and supplier cost structures outside of bankruptcy.

"GM also faces the near-term challenge of completing the sale of GMAC and resolving the current SEC investigations into various accounting matters," Moody's said in a statement.

The company's operating profile continues to be pressured by declining U.S. market share, the ratings agency said.

Moody's cut GM's long-term senior unsecured rating by one notch to "B2," five levels below investment grade, from "B1." The outlook is negative, meaning an additional cut is likely over the next 12 to 18 months.

Moody's also affirmed the senior unsecured rating on GM's finance arm, General Motors Acceptance Corp. at "Ba1," one level below investment grade.