Audit fees have increased over the year with an average growth of 7.9% at the Big Four, which is expected to continue as new audit rules come into effect forcing the largest audit firms to hive off their audit businesses into discrete entities to avoid risks of conflicts of interest. Meantime, revenue at challenger firms was up 20% from audit contracts.

Despite the growth in FTSE 350 market share, all FTSE 100 companies are audited by a Big Four firm with little sign of competition at the top end of the market.

Challenger audit firms include BDO, Grant Thornton, RSM and Mazars which all reported growth in audit income.