The Big Four accounting firms—PwC, Deloitte, EY, and KPMG—have deliberately reduced their presence on London's Alternative Investment Market (AIM), opening the door for mid-tier auditors to capture new business. According to a report by Adviser Rankings, the number of FTSE AIM 100 companies audited by the Big Four has plummeted from 49 three years ago to just 30 today. In the FTSE AIM UK 50 index, their share fell from 58% to 42% over the second quarter of 2026.
Mid-Tier Firms Seize Opportunities
Mid-tier audit firms have capitalised on this shift. BDO, for example, won high-profile mandates directly from the Big Four, including oil and gas company Serica Energy PLC (previously audited by EY) and agricultural group Camellia PLC (previously audited by Deloitte). Grant Thornton, now backed by private equity, added the most new clients in the FTSE AIM 100 during Q2 2026, climbing to fifth place. In the FTSE AIM UK 50, Grant Thornton doubled its client count to tie with KPMG and RSM UK in fourth place.
PKF Littlejohn and MHA Gain Traction
AIM specialist PKF Littlejohn extended its lead over BDO to 21 clients, reaching a total of 90 AIM mandates—its highest count in two years and the highest overall tally recorded by any auditor since January 2025. Meanwhile, London-listed MHA audit services added two new clients, entering the total AIM top 10 for the first time and jumping from eleventh to ninth place.
Reasons Behind the Big Four's Retreat
The Big Four's withdrawal stems from a series of high-profile audit failures that led to substantial fines from the Financial Reporting Council (FRC). To protect their reputations and avoid future penalties, they have systematically shed higher-risk clients. AIM, as a junior growth market, carries a greater risk of failure than main market blue chips. Coupled with increased FRC pressure for improved audit quality and rising audit costs, many mid-cap and small-cap AIM companies find Big Four audits unaffordable.
In contrast, the Big Four continue to dominate the FTSE 100 audit market. A recent report revealed that Deloitte, KPMG, and PwC are now locked in a three-way tie at the top of the FTSE 100 rankings for the first time in nearly eight years.
The shift in the AIM market represents a significant realignment in the UK audit landscape, with mid-tier firms increasingly competing for and winning business that was once the exclusive domain of the Big Four.
