Research carried out by Accountancy Age’s sister title, Financial Director, shows that BDO now audits seven companies in the FTSE 100 and FTSE 250.

But the mid-tier challenge to the Big Four dominance of major audits has otherwise faded. Brit Insurance dropped Mazars for Ernst & Young; Group 4 Securicor switched from Baker Tilly to KPMG; and iSoft dropped Robson Rhodes, leaving the FTSE 250 index at the same time.

BDO’s seven audits mean the mid-tier now accounts for just over 2% of the companies in the survey.

The falling away of the mid-tier comes at a time when regulators are doing their best to encourage them to compete for more high-profile work, as the competitiveness of the audit market comes under government scrutiny.

The research leaves out investment trusts, some of which do still have mid-tier auditors.

BDO made £6.7m in audit fees from its activities, £5.2m of which came from its lone FTSE 100 client Partygaming. Its other audits are Countrywide, 888 Holdings, Derwent Valley Holdings Restaurant Group, RPS Group and SCI Entertainment.

Audit fees for FTSE 100 clients are up by 14% on last year, according to the survey, with FTSE 250 audits up by 5%.