Credit ratings of DP World, the ports operator eyeing a dual listing on the London stock exchange, have been upgraded a notch by Moody's, citing the firm's operating performance and industry fundamentals, Reuters has reported. The port operator was upgraded to investment grade with 'Stable' outlook. "Today's one-notch upgrade of DP World into investment grade reflects Moody's view that the company has achieved credit metrics that are commensurate with a higher rating category and are likely to be maintained over the medium term," Franck Nowak, associate analyst at Moody's in Dubai said in a statement.