The business model of the Big Four accounting firms is under attack from the European Commission, which is pushing for tough rules that would force the firms to abandon their consultancy businesses and share audit work with smaller rivals.

A draft regulation, seen by the Financial Times, aims to transform the accounting sector in the wake of the financial crisis and restore “trust” in financial reporting. It has the backing of Michel Barnier, internal market commissioner, whose officials have decided the audit world is in the grip of an oligopoly.

Under the plans, which are to be unveiled in November, companies with balance sheets greater than €1bn would be forced to hire two auditors to conduct a “joint-audit” of their books, including at least one firm outside the Big Four of Deloitte, PwC, Ernst & Young, and KPMG.