SHELL’S external auditors, KPMG and PricewaterhouseCoopers, have come under scrutiny over allegations that they were warned by the oil giant’s group reserves auditor in 2002 that reporting of Shell’s reserves was questionable.

Internal reports critical of the Anglo-Dutch group’s reserves booking were allegedly sent to the outside auditors by Anton Barendregt, a former reserves auditor.

Shell revealed in January that a fifth of its oil and gas reserves were misreported, a scandal that has led to the removal of three senior directors, including the chairman, Sir Philip Watts.

KPMG and PricewaterhouseCoopers yesterday declined to confirm that they had received Mr Barendregt’s reports, written in early 2002 and 2003, which revealed his concern that the bookings did not comply with SEC rules.

In his reports, Mr Barendregt was allegedly concerned that 1 per cent of the oil and gas reserves were not compliant and he raised concerns that bonuses paid to staff for boosting reserves were a threat to the integrity of reserve estimates.

KPMG said yesterday that it was confident of the integrity of its audit.

In a report on the reserves scandal by Davis Polk & Wardwell to Shell’s audit committee, Mr Barendregt states that when he signalled conflicts with SEC rules to his superiors he was overruled by the Reserves Committee and exploration executives.

He said: “With hindsight, I should have been more forceful . . . it would probably have cost me my job in those days, but I should have.”

Shell’s auditors had no direct responsibility for auditing the reserves statement, not part of the company’s statutory accounts, but questions are being raised as to how far they had a duty to ensure the integrity of the process.

Leading accountancy firms are reluctant to tackle reporting on oil and gas reserves as they lack the engineering and geological skills needed to interpret the data.

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