Public company and bank audits conducted around the globe by units affiliated with the world's six largest accounting firms are persistently riddled with flaws, a group of international regulators have found.

The finding, released in a survey by the International Forum of Independent Audit Regulators (IFIAR), raises major policy questions about whether enough has been done by global regulators to improve audit quality since the 2007-2009 financial crisis.

Leading up to the crisis, many publicly traded banks portrayed a rosy financial picture of their corporate books, only to later suffer massive losses on subprime mortgage securities in their portfolios.

Critics have questioned why independent auditors tasked with reviewing the accuracy and quality of public company financial reporting failed to spot the problems sooner.

"The high rate and severity of inspection deficiencies in critical aspects of the audit, and at some of the world's largest and systemically important financial institutions, is a wake-up call to firms and regulators alike," said Lewis Ferguson of the Public Company Accounting Oversight Board, the body that polices auditors in the US.

"More must be done to improve the reliability of audit work performed globally on behalf of investors."

The global survey on audit performance comes at the end of a three-day summit in Washington, D.C., that included audit regulators from around the globe.

Together, those 50 regulators comprise the IFIAR - a coalition that was formed in 2006 to improve information-sharing and coordination.

The findings discussed in Thursday's survey stem primarily from inspections conducted at firms affiliated with the six largest accounting firms in 2013.

That includes the "Big Four" - PricewaterhouseCoopers , KPMG, Deloitte and Ernst & Young , as well as BDO and Grant Thornton.

The survey looked at inspection results for audits of public companies and large financial institutions considered "systemically important" to the global economy.

It also looked at how well internal quality controls fare at audit firms themselves.

With public company audits, regulators found problems related to auditing fair value measurements, internal control testing and procedures used to assess how financial statements are presented.

The regulators also said that audits of systemically important financial firms often had deficiencies stemming from allowances for loan losses and loan impairments and the auditing of investment valuation.

In all of these examples, IFIAR said auditors did not always obtain sufficient evidence to support their audit opinions.

As for audit firms, the regulators said they routinely encountered problems with independence and ethics, among other things.

The survey is IFIAR's second. Last year, similar types of problems were flagged, though IFIAR says the survey itself does not provide an adequate basis to make a year-to-year comparison.

Regulators in the US and Europe have been exploring ways to improve audit quality since the 2007-2009 financial crisis.

Last week, the European Union approved some of the world's toughest new rules for accountants, after auditors gave banks a clean bill of health before they were bailed out by taxpayers.

Those rules would prevent accounting firms from also auditing the books of a public company client for more than 20 years, a reform designed to bolster auditor independence and end cozy relationships between accountants and company management.

The PCAOB has given up exploring a similar reform in the United States, after major business groups and accounting firms lobbied fiercely against it.

However, the board is working toward completing other reforms. One plan would require auditors to tell investors more details about "critical audit matters" they encountered during a review of the company books.

Another plan calls for auditors to disclose the names of individual partners who work on company audits, in an effort to hold them more accountable. -Reuters