An Australian court has ruled that ratings agency Standard & Poor's misled investors when it gave the highest investment rating to complex and risky securities before the financial crisis.

The Federal Court ruling is the first in the world on a rating agency's liability for investors' losses and may have implications for Fitch and Moody's.

Judge Jayne Jagot ruled that S&P and ABN Amro were liable for advice given and distribution of the derivatives to several Australian local governments. 

The value of the derivatives, which were rated AAA by S&P, tumbled during the crisis.

The court said a "reasonably competent" rating agency could not have given a triple A rating to the securities, which were described as "grotesquely complicated".

S&P plans to appeal the decision, which could cost it and the bank $31m in damages.