Big Four’s AIM exodus accelerates as mid-tier firms seize mandates
Mid-tier audit firms scooped up new clients as the Big Four giants executed an intentional retreat from London’s junior market.
Three years ago, PwC, Deloitte, EY, and KPMG audited nearly half of the FTSE Alternative Investment Market (AIM) 100 index with 49 clients combined. But today, the market share has collapsed to 30 clients, according to a new report by Adviser Rankings.
In the FTSE AIM UK 50, the Big Four’s share of constituents has dropped from a dominant 58 per cent three years ago to 42 per cent over the second quarter of 2026.
Mid-tier BDO has snatched high-profile mandates directly from Big Four rivals, including oil and gas firm Serica Energy PLC from EY and agricultural group Camellia PLC from Deloitte.
Freshly equipped with private equity funding, Grant Thornton added the most new clients in the FTSE AIM 100 during Q2, climbing to fifth place, and doubled its client count in the FTSE AIM UK 50 to tie in fourth place alongside KPMG and RSM UK.
AIM specialist PKF Littlejohn extended its client lead over BDO from 12 to 21 clients. It also reached 90 total AIM mandates, as the firm posted its highest client count in two years and the highest overall count seen for any auditor since January 2025.
While London-listed MHA audit services added two new clients to break into the total AIM top ten for the first time, jumping from eleventh to ninth place.
Audit failures reshaped the market
Following several high-profile audit failures resulting in critical fines from the watchdog, the Financial Reporting Council (FRC), the Big Four have actively purged their client rosters of higher-risk companies in order to protect reputations and avoid future regulatory fines.
As AIM is inherently a junior, growth-focused market, it carries a higher risk of failure than main market blue chips.
In addition, the increased pressure from the FRC for higher audit quality, combined with rising audit costs, has priced out many mid-cap and small-cap AIM companies. Instead, the Big Four dominate the FTSE 100 audit market.
A recent report revealed that, for the first time in nearly eight years, Deloitte, KPMG, and PwC were locked in a three-way tie at the top of the FTSE 100 audit rankings.