MEXICO CITY, July 21 (Reuters) - Ratings agency S&P Global on Thursday affirmed Honduras' sovereign credit rating at 'BB-/B' and lowered its outlook to "negative" on the expectation of fiscal deterioration.

"The negative outlook reflects our view that there is at least a one in three chance that we will downgrade Honduras in the next six to 18 months, if the fiscal deterioration worsens the sovereign debt burden beyond our expectations," S&P Global said in a statement. (Reporting by Noe Torres; Writing by Kylie Madry; Editing by Anthony Esposito)