Mid-tier audit firms have gained ground in London’s junior market as the Big Four heavyweights deliberately withdraw from the FTSE Alternative Investment Market (AIM). PwC, Deloitte, EY, and KPMG audited almost half of the FTSE AIM 100 index with 49 clients between them three years ago.
Today, their market share has slumped to just 30 clients, according to a new report by Adviser Rankings.
In the FTSE AIM UK 50, the Big Four’s share of constituents has fallen from 58 per cent three years ago to 42 per cent over the second quarter of 2026.
BDO has seized high-profile mandates directly from Big Four rivals, among them oil and gas company Serica Energy PLC from EY and agricultural group Camellia PLC from Deloitte.
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Grant Thornton added the greatest number of new clients in the FTSE AIM 100 during Q2, rising to fifth place, and doubled its client count in the FTSE AIM UK 50 to draw level in fourth place alongside KPMG and RSM UK.
Following a series of high-profile audit failures that resulted in substantial fines from the watchdog, the Financial Reporting Council (FRC), the Big Four have systematically stripped their client lists of higher-risk companies in a bid to safeguard reputations and prevent future regulatory penalties.
As AIM is fundamentally a junior, growth-oriented market, it presents a greater risk of failure than main market blue chips.
The heightened pressure from the FRC for improved audit quality, coupled with escalating audit costs, has made audits unaffordable for many mid-cap and small-cap AIM companies.
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Meanwhile, the Big Four dominate the FTSE 100 audit market, with Deloitte, KPMG, and PwC recently locked in a three-way tie at the top of the FTSE 100 audit rankings.
PKF Littlejohn extended its client lead over BDO from 12 to 21 clients, reaching 90 total AIM mandates — the firm’s highest client count in two years and the highest overall tally recorded by any auditor since January 2025.
MHA audit services added two new clients to break into the total AIM top 10 for the first time, leaping from eleventh to ninth place.
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