In a draft of regulation due to be published in November, EU internal markets commissioner Michel Barnier argues that the recent financial crisis "highlighted weaknesses in the statutory audit" and proposes a series of measures that would impose significant changes to the industry.
The proposals broadly push for greater competition within the industry, and large companies would be required to undergo a joint audit by two auditors with at least one coming from outside the Big Four. The proposals also call for a ban that would restrict auditors from providing consultancy services to companies and clients they audit, or even be banned from consulting. This is in line with what Barnier sees as a "conflict of interest" when audits charge high consultancy fees for the books that the check.
This could possibly be the biggest shake up for the industry since the collapse of Enron that subsequently led to the 2002 Sarbanes-Oxley Act.
Other proposals include the requirement that companies change their auditors after every nine years.
The Big Four firms include KPMG, Ernst & Young, PricewaterhouseCoopers and Deloitte LLP.
In terms of numbers, the Big Four perform 99 percent of audits for all FTSE 100 companies, and 95 percent of those on FTSE 250 companies, and more than 85 percent of big companies in a majority of the EU nations.