Moody's yesterday became the first credit rating agency to signal a tougher line on downgrades of enfeebled companies following Enron's collapse.

The agencies have been criticised for deferring downgrades of the energy trader's debt until it became clear that a rescue bid from rival Dynegy was doomed.

The downgrades triggered a chain reaction of debt repayment and refinancing that forced the company to seek protection from its creditors last Sunday.

Moody's said in future it would take into account the ability of companies to handle the "unintended consequences of rating triggers". That could mean downgrading the company earlier in the process, said Ms Pamela Stumpp, Moody's senior credit officer.