Investors need to carry out more credit work of their own before buying debt rather than relying on credit ratings, debt market participants said. The three major rating agencies - Moody's Investors Service, Standard & Poor's and Fitch Ratings - have come under fire in the wake of sharp downgrades on structured products with exposures to the US subprime market to "junk" from triple-A.

The recent Financial Stability Forum report, commissioned by the Group of Seven nations, recently called for tougher standards for ratings companies. US Securities and Exchange Commission chairman Christopher Cox said on Wednesday the SEC would soon propose more rules to police credit rating agencies.

But speakers at a conference in London, organised by the Centre for the Study of Financial Innovation, said the role of rating agencies was limited.

"Investors have to do more homework. Ratings are an interesting benchmark but they are just an opinion," said Joe Biernat, head of credit research at European Credit Management.

Away from the troubled field of structured finance there was praise for the role of ratings agencies.