(Reuters) - Detroit's already low junk credit rating was downgraded to C by Fitch Ratings on Tuesday over concerns the beleaguered city may miss a debt service payment.

Fitch lowered the city's unlimited tax general obligation rating on about $511 million of bonds to CCC from B and cut the rating on about $453 million of limited tax GO bonds to CC from B.

The rating on Detroit's $1.5 billion of pension obligation certificates of participation was downgraded to CC from B with a warning that the rating could fall to the lowest level D if the city misses a debt service payment.

(Reporting by Karen Pierog)