Who Audits Public Companies – United Kingdom 2021 

Overall, the Big Four firms retained 83.1% of the market share for the major UK indices in 2021. This is a slight decline from the 84.3% market concentration seen in 2020. While PwC signed opinions for 26.6% of major LSE index-listed firms, the rest of the Big Four dropped to 56.5% of the market. Relative to 2020, this drop represents a slight decline for KPMG and EY and a slight increase for Deloitte. 

Outside the Big Four, BDO remained the leading auditing firm with an 8.7% share of the market. Grant Thornton declined to 3% of the market share. The remainder were audited by eight firms, with 1% or less of the market share held by each. 

2021 Auditor Market Share

FTSE 100 

The Big Four dominated the market share for the FTSE 100 as of 2021. The distribution of auditors narrowed to become nearly even among them. In comparison to last year, BDO has broken into the market share with a 1% stake in the index. KPMG remained in the lead in audits of the FTSE 100, conducting 27% of the audits. PwC’s share increased by 1%, leaving them with a 26% share overall as the second-largest share of the market. EY dipped by 2%, forfeiting the third-largest ranking to Deloitte, who experienced a 1% increase as well.

FTSE 250 

PwC remained the leading auditing firm of the FTSE 250 with a market share of 31%, a 1% decrease from the previous year. KPMG experienced the largest decline out of the Big Four, ranking last with only 18%. In 2020, KPMG was the second-leading auditing firm with 21%. Deloitte secured 20% of the market share, the same as last year. EY was able to edge out KPMG by holding 1% more and covering 19% of the FTSE 2500 index. In total, the Big Four signed 88% of the audits for the index.  

In 2021, the index saw market shares for non-Big Four firms continue their slow growth. BDO’s market share increased by contributing 8% in 2021 of the FTSE 250 market share versus 6% in 2020. In addition, Grant Thornton and RSM’s shares both slightly increased as well. The newest entrant to the market share in 2021 is Mazars, holding 1% of the market share. Now that Bridgepoint Group PLC [BPT] has appointed the auditing firm during its admission to the London Stock Exchange.


Consisting of the 50 largest firms on the LSE’s AIM, the audit market for the FTSE AIM UK 50 showed some fluctuation. The Big Four’s market share contribution declined from 66% in 2020 to 57% in 2021. Equally, each Big Four firm audited seven companies compared to BDO auditing 12 companies. BDO’s market share increased from 20% in 2020 to 25% in 2021, becoming the leading auditing firm for the FTSE AIM UK. In 2021, the non-Big Four auditing firms doubled, collectively increasing to 8% of the market share. Crowe, UHY, and PKF we also new entrants to the market this year for the FTSE AIM UK 50 index.  


As it encompasses companies on the FTSE AIM UK 50 as well as the 50 next-largest AIM companies, the FTSE AIM 100 showed notable movements in the audit market relative to 2020. Markedly, BDO has surpassed the Big Four, consuming 20% of the market share in 2021 compared to 15% in 2020. PwC experienced the largest decrease in market share, from 23% in 2020 to 16% in 2021. KPMG and Deloitte tied third in the ranking, auditing 13 companies each. Grant Thornton declined by 2%, while Crowe, PKF, and RSM all gained a larger slice of the market, respectively. Crowe and PKF each held a 5% market share, and RSM, 3%. All other firms held a 1% market share.

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