It is the first time the global accounting network has retained number one spot in consecutive years, beating rival PwC by $248m.
PwC, however, is yet to see the impact of the acquisition of global consultancy Booz & Co, which was finalised in the last few months of its 2014 financial year.
While the gap between the top two firms has narrowed, the gap between them and third ranked EY has been increasing - from $1.9bn in 2004 to $6.5bn in 2014. EY reported fee income of $27.3bn and fourth ranked KPMG $24.8bn in 2014.
All of the Big Four have increased fee income. In 2013 Deloitte reported fee income of $32.4bn, just ahead of PwC with $32bn. EY came in third with $25.9bn, followed by KPMG with $23.4bn.
In addition, the gap between the Big Four and the nearest competitor, BDO, has widened by over $7bn in the past decade from $10.6bn in 2004 to $17.8bn in 2014, according to the survey of global accounting firms conducted by International Accounting Bulletin (IAB).
The Big Four controlled two thirds of global accounting market share in 2014 with a combined $120.2bn in fee income.
In total 52 top international accounting networks and associations earned a combined $181.7bn in fees in 2014, up 6% year-on-year.
Advisory and strategic acquisitions have driven growth in the past few years. IAB data shows that since 2004 the Big Four combined have increased their advisory revenues by over $33bn while audit and accounting revenues increased by $14bn and tax work revenues by $10.2bn in the same time period.
M&A was one of the main drivers of growth for the largest firms, with deals such as the PwC acquisition of Booz & Co., EY’s acquisition of The Parthenon Group as well as its merger with KPMG Denmark, and KPMG’s many consultancy and analytics business acquisitions in the US and Europe.
The survey has warned of the dangers of market fragmentation caused by the soon to be implemented EU audit reform (set for June 2016) as member states are left to mull over 83 options presented to them in the regulation and directive.