The president of Standard & Poor's, Kathleen Corbet, has stood down with immediate effect amid questions over the role of credit rating agencies in the current market turmoil.

Ms Corbet said she was leaving the world's biggest rating agency, part of the American publisher McGraw-Hill, to "pursue other opportunities". She had been with the company for three years.

McGraw-Hill has replaced Ms Corbet with Deven Sharma, executive vice-president of investment services and global sales at S&P since last November.

A company spokesman stressed that Ms Corbet's departure was not related to criticisms of S&P's ratings. He declined to comment on whether she would be leaving with a payoff, saying this was confidential.

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"Kathleen stepped down to pursue other professional opportunities and to spend more time with her family, he said. He noted that 40 per cent of the group's revenues from ratings comes from outside the US and that Mr Sharma would be instrumental in growing the business.

However, it comes as credit agencies, including S&P and its rival Moody's, face questions over their ability to spot financial market crises.

European politicians, including Angela Merkel, the German chancellor and President Nicolas Sarkozy of France, have called for rating agencies to be forced to be more transparent about their rating actions.

Charlie McCreevy, EU financial services commissioner, has also expressed concerns. The EU is considering legislation in this area.

Agencies such as S&P and Moody's assigned top-notch AAA ratings to highly geared investment vehicles, which have seen their values collapse in recent weeks following soaring delinquencies among US sub-prime mortgage borrowers. The two agencies have now downgraded hundreds of vehicles.

Critics also argue that S&P and Moody's, as well as other agencies, face an inherent conflict of interest, in that many of their clients issue securities that are rated by its analysts.

In their defence, the agencies argue that they can only operate within the boundaries of what the entities they rate tell them. They defend their ability to manage perceived conflicts and stand behind the integrity of their analysts.

Both S&P and Moody's were criticised four years ago when Parmalat, the Italian dairy group, collapsed into insolvency with debts of €14 billion. S&P, which downgraded Parmalat by six ratings notches in one day, was criticised for not spotting problems with Parmalat's debt.

Mr Sharma, a former partner at the Booz Allen Hamilton consultancy, joined McGraw-Hill in 2002 and served in several strategic positions, including working on acquisitions, until being assigned to S&P.

Harold McGraw III, the chairman, president and chief executive of the McGraw-Hill Companies, said: "The continual expansion of the world's capital markets presents enormous growth opportunities for Standard & Poor's, and Deven's strategic thinking and deep understanding of global financial markets will help us continue to identify and benefit from these opportunities."

Mr Sharma will report to Mr McGraw.

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