Key facts
- The Big Four's combined client count on the FTSE AIM 100 has fallen to 30, down from 49 three years ago.
- Mid-tier firms BDO and Grant Thornton have gained significant new audit mandates on the AIM market.
- The Big Four are intentionally retreating from higher-risk AIM companies due to regulatory pressure and audit failures.
- The Big Four now primarily focus on auditing FTSE 100 companies.
The dominance of the Big Four accounting firms—PwC, Deloitte, EY, and KPMG—in auditing companies listed on London's Alternative Investment Market (AIM) has significantly waned. Three years ago, these firms audited nearly half of the FTSE AIM 100 index constituents, but their combined market share has now fallen to just 30 clients, according to a report by Adviser Rankings. Similarly, their share of the FTSE AIM UK 50 has dropped from 58% to 42% in the second quarter of 2026.
This strategic retreat by the Big Four has created opportunities for mid-tier firms. BDO has successfully acquired high-profile clients, including Serica Energy PLC from EY and Camellia PLC from Deloitte. Grant Thornton, bolstered by private equity funding, added the most new clients in the FTSE AIM 100 during the second quarter and doubled its client count in the FTSE AIM UK 50, placing it in fourth position alongside KPMG and RSM UK. PKF Littlejohn has further extended its lead over BDO, reaching 90 total AIM mandates and achieving its highest client count in two years. MHA audit services also made gains, breaking into the AIM top ten for the first time by moving from eleventh to ninth place. The shift is attributed to several factors, including several high-profile audit failures that resulted in significant fines from the Financial Reporting Council (FRC). To protect their reputations and avoid future regulatory penalties, the Big Four have been actively pruning their client lists, particularly those in the inherently higher-risk AIM market. Furthermore, increased demands for audit quality from the FRC, coupled with rising audit costs, have made Big Four services less accessible for many mid-cap and small-cap AIM companies. Consequently, the Big Four have consolidated their focus on auditing larger companies, now dominating the FTSE 100 audit market.