BDO's merger with PKF has seen the combined firm take the top slot as the leading auditor for AIM, replacing Grant Thornton, according to the latest research by Adviser Rankings.

Its Q2 2013 quarterly Corporate Advisers Rankings Guide published in association with Crowe Clark Whitehill, shows that BDO in first place in terms of numbers of AIM clients (up to 159 from 132), while GT's client list has dropped from 158 to 149. UHY Hacker Young moves up from 11th to ninth place in the rankings.

BDO has also increased the number of FTSE 250 clients it audits, up from seven in the previous quarter to nine currently, which has produced a 32% increase in aggregate market capitalisation. However, overall rankings for this sector remained unchanged, with Deloitte having the most FTSE 250 audits, followed by KPMG, PwC and Ernst & Young.

When it comes to audits of the biggest UK companies, BDO continues to have just one FTSE 100 audit client. PwC, extended its leadership of the table by gaining a client, as did Deloitte, which tied with KPMG for second place. Fourth-placed EY has 15 FTSE 100 audits.

In terms of market capitalisation of FTSE 100 audit clients, PwC is in the number one slot at £894,678, almost double second-placed KPMG (£ 477,740).

Taking the stock market as a whole, KPMG has replaced PwC as the leading auditor in terms of client numbers (367). However, PwC's client base of 363 remains substantially ahead by aggregate market capitalisation (£1.26m compared to KPMG's £706,702). BDO moved up one place to fifth both in terms of client numbers and market capitalisation.