LONDON -- After a golden decade of improvement, credit ratings for a swathe of developing economies risk falling back to "junk," with huge potential costs for up to a tenth of outstanding emerging market bonds.

Many mainstream investment and pension funds have rules preventing them from holding debt unless it is classified as investment grade by at least two of the big ratings agencies, and a number of countries are at risk due to problems ranging from tumbling commodity export prices to political instability.

Russia this week became the first of the major economies to lose its investment grade status from Standard & Poor's, falling out off the top ratings category for credits deemed to have a low risk of default for the first time in a decade.

If Moody's and Fitch follow, conservative investors barred from owning junk securities must sell their holdings. JPMorgan estimates this means they may ditch $6 billion in Russian government rouble and dollar debt.

Russia may have company. Almost $260 billion worth of sovereign and corporate bonds -- nearly a tenth of outstanding emerging market (EM) debt - is in danger of being relegated to junk, according to David Spegel, head of emerging debt at BNP Paribas, who calls such credits "falling angels."

What's more, almost $1 trillion of debt is rated BBB or BBB minus - the two lowest investment grade ranks after which junk or "high yield" status awaits.
"After a year of political upheaval and collapsing commodity prices, the sky is alight with EM falling angels," Spegel said.

In 2010, for the first time, a majority of bonds in the EMBI Global index of emerging market debt became investment grade . But now a fifth of emerging market governments rated by S&P carry negative outlooks; the agency calls emerging markets the "weak link" in the global ratings picture.

If there is a series of downgrades, the entire index could shift lower again, Spegel warned, adding: "The EM benchmark index is at risk of becoming a falling angel."

Ratings models compiled by analysts at Bank of America/Merrill Lynch show downgrade risks in Brazil, Russia, Turkey, South Africa and Indonesia.
Some of these, such as energy importers which benefit from falling oil prices and countries making economic reforms, may avoid relegation.

However, ratings tend to move up and down in tandem, BofA noted. It cited negative credit revisions in the 1980s, upgrades in the early 1990s, downgrades in the late 1990s and another round of upgrades this century. Two-thirds of emerging economies are investment grade, up from 42 percent a decade ago, it added.