The closely watched annual survey, published by the International Accounting Bulletin, showed that the "Big Four" accounting firms – PwC, Deloitte, KPMG and Ernst & Young – took 67% of the total $165.4 billion (€121.8bn) in fees which the sector earned in 2012, little changed from 2011.
Deloitte maintained its position as the second largest firm, just $210 million (€154.7mln) in fees behind PwC's $14.9 billion (€11bn). Growth in income at the big firms collectively slowed to 6% from 8% in 2011.
"There have been almost no year-on-year changes in market share within the Big Four," the survey said.
Still, the fees gap between third-ranked Ernst & Young and fourth-placed KPMG rose sharply in 2012 to $1.4 billion (€1.03bn), compared with only $170 million (€125.2mln) in 2011.
The sector is braced for potentially radical changes.
The UK Competition Commission is investigating whether there is enough competition in auditing large firms and will report next month with possible reforms.
Meanwhile the European Union is working on a draft law to require a periodic "rotation" or mandatory switching of auditors by clients, an idea U.S. regulators are also looking at.
The survey said client behaviour is already changing in anticipation of these reforms.
More and more public companies have told CEOs of accounting firms that they were thinking more seriously about putting their audit requirement out to tender, examining their auditors, and starting to consider smaller accounting firms for their tax and advisory work, the survey said.
Outside the Big Four, BDO remains in fifth place in the rankings, followed by Grant Thornton, RSM, Baker Tilly International, Crowe Horwath International and Nexia.
