Brian Clarkson, the man behind Moody’s Investors Services push into high-risk mortgage-backed securities, has become the second top executive to leave a leading ratings agency in the wake of a credit crunch that has wiped nearly $320 billion worldwide off banks’ bond holdings.

Mr Clarkson, 52, whose departure is characterised as a voluntary retirement, will be succeeded by Michel Madelain, also 52, who presently runs Moody’s corporate and government bonds unit. Mr Clarkson will retire at the end of July.

Like its key competitors, Standard & Poor’s and Fitch, Moody’s is the subject of numerous investigations into whether it fuelled the housing boom – and the credit crunch that resulted when it bust – by assigning ratings to mortgage-backed bonds that were too high.

In particular, the agencies stand accused of lulling investors into a false sense of security and perpetuating the housing boom by giving the top AAA-rating to collatoralised debt obligations (CDOs) – often high-risk and unfathomable pools of mortgage-backed assets.

Hundreds of these securities have been downgraded in recent months, often by several notches, as many slumped to less than one tenth of their value.

As the head of the Moody’s unit that oversaw mortgages and other so-called structured finance products, such as CDOs, Mr Clarkson changed the methodology for rating some mortgage bonds. This led to higher ratings and helped Moody’s to increase its share of the market.

Mr Clarkson, a lawyer who joined the group in 1991 as an analyst, was promoted last year to president and chief operating officer of Moody’s Investors Service, the ratings agency that makes up the bulk of the organisation.

Raymond McDaniel, chief executive of Moody’s Corporation, of which the Moody’s Investors Service ratings unit is the main business, said: "Challenging credit-market conditions, combined with Moody’s role and function in those markets, have created scrutiny and criticism from numerous external sources about various aspects of our business."

Brad Hintz, an analyst at Sanford Bernstein, said: "This shows that Moody’s is taking action to repair its credibility. It says ‘we are tightening up’."

The departure of Mr Clarkson follows the replacement last August of Kathleen Corbet as president of Standard & Poor’s, amid mounting criticism for its role in the unfolding sub-prime mortgage crisis.

The ratings agency said that Ms Corbet was leaving to pursue unspecified opportunities and her role was taken over by Deven Sharma, a senior executive at S&P.

Warren Buffett, the world’s richest man, appeared to criticise the bond ratings industry at his Berkshire Hathaway investment group’s annual shareholder meeting in Omaha, Nebraska at the weekend.

Mr Buffett said that most of the bonds his new securities underwriter had insured in its first full quarter of operation were already insured by a top-rated underwriter.

"It tells you something about the state of AAA when they are paying us to write business that is already insured, Mr Buffett said.

Berkshire Hathaway owns one fifth of Moody’s shares, a holding that is being investigated as part of a broader ratings probe by Connecticut’s attorney general, over a potential conflict of interest with his ownership of a bond insurer.

Mr Buffett has denied that there is any conflict of interest between his part-ownership of Moody’s and his control of Berkshire Hathaway Assurance, his bond insurer.

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