Big Four accounting giant EY has brought in Magic Circle firm Slaughter and May as it seeks to challenge the competition watchdog's bid to introduce sweeping legislative reforms to the auditing industry.

The Competition and Markets Authority (CMA) last month proposed new regulations for the market to tackle "inadequate choice and competition", due to the industry's heavy reliance on each of the Big Four firms.

Among other things, the CMA proposed to: separate audit from consulting services; to introduce a mandatory 'joint audit' to enable firms outside the Big Four to develop the capacity needed to review the UK's biggest companies; and introduce statutory regulatory powers to increase accountability of companies' audit committees.

The auditing giant has now turned to Slaughters as it mounts a challenge to the CMA's intervention, a person with knowledge of the matter confirmed.  

The proposed changes drew criticism from EY, which argued that there was "lack of evidence" for the regulator's findings. It said it "fundamentally disagreed" with the CMA's proposal, and that such measures "would undermine audit quality by reducing our ability to draw on critical skills, capabilities and investment, and diminish the resilience of the audit business".