An independent investigation into Shell's overstating of oil reserves called for the company's legal function to be given a beefed-up role this week.

The report by US law firm Davis Polk & Wardwell blamed the 'control failures' at the petroleum giant in part on an inadequate understanding of the disclosure rules of US watchdog, the Securities & Exchange Commission.

Potentially material disclosure and compliance issues were discussed at meetings of the company's committee of managing directors (CMD) without advice from lawyers with securities law or corporate governance expertise, it said.

'The visibility of the internal legal function at the highest levels should be increased and this should start by providing for attendance by Shell's legal director at meetings of CMD, conference [Shell's strategic review body] and the parent company boards,' Davis Polk said.

The US firm's investigation - during which it interviewed 90 witnesses - recommended that serious consideration should be given to making the legal director, Beat Hess, chairman of the company's disclosure committee.

It added that the general counsel of the various Shell businesses, who attend executive committee meetings, should have an aim of identifying disclosure issues for consideration at a higher level.

The report said that 'to remove any reporting ambiguities', all lawyers at Shell, including the company secretaries of the parent companies, should 'unambiguously' report to Mr Hess.

Davis Polk also called on the legal function actively to identify and address training needs throughout the company in areas of disclosure, reporting obligations and corporate governance.

The investigation was commissioned by Shell's group audit committee (GAC) in February to examine why 20% of its oil and gas reserves - 3.9 billion barrels - had to be recategorised after being overstated as proven rather than probable.

The crisis has led to the departure of three top executives, including chairman Sir Philip Watts.

Shell's UK general counsel Richard Wiseman said: 'We have already - quite apart from the facts underlying the report - identified substantial things which we ought to be doing in relation to compliance that required a higher profile as far as the lawyers are concerned.

'Last year, quite independently of the report, we reviewed the global legal structure and all lawyers now have direct lines to Beat Hess.'

A Shell statement said the company's boards fully accepted the findings of the report and would act promptly on its recommendations.

The company also said it would accelerate the corporate governance review announced last month.