Shares of Sino-Forest (TRE.TO: Quote) fell more than 15 percent on Tuesday after rating agency Standard & Poor's once again lowered the credit rating of the Chinese forestry company accused of fraud.

The Toronto-listed company's shares have tumbled more than 75 percent since the beginning of June, when short-seller Carson Block said it fraudulently exaggerated the size of its forestry assets. Sino-Forest has denied any wrongdoing and it has asked a committee of its independent board members to look into the allegations.

The company's internal probe was originally expected to end around mid-September, but last week, Sino-Forest said the review would not wrap up until the end of the year.

The delay in part prompted S&P on Tuesday to cut its rating on the company's debt to B from B+.

We believe the delay in the findings of an independent committee's investigation into fraud allegations is negative for Sino-Forest's credit profile, said the rating agency in a statement.

S&P said it may even withdraw or suspend its ratings on the company entirely, if the company delays its quarterly results announcement, or if the investigation is extended again.

The latest cut from S&P follows an earlier ratings cut on June 30, when the firm lowered its long-term corporate credit rating on Sino-Forest to B+ from BB.

Shares of Sino-Forest dropped 13.3 percent at C$4.34 at 1255 ET on the Toronto Stock Exchange.