Lindsay and his former Billetts directors Tim Keen and Neil Spencer have created a joint venture company, Slik Media, in partnership with the London-based auditor Fairbrother Lenz Eley.
Fairbrother has taken a minority shareholding in the business, with the remaining shares held by the three founding partners. The deal has given Slik Media access to Fairbrother's research system.
The creation of the company could be seen as a threat to Billetts' stranglehold on the market but its chairman, John Billett, rejected this view.
He said that, with a staff of around eight client-facing auditors, the Billetts Midlands office handled some of his biggest advertiser clients - it is understood that VW Group, Nationwide and PepsiCo are all serviced through Birmingham.
Billetts recently centralised back-office functions, such as accounts and computing, into its Midlands office.
Lindsay, Keen and Spencer left Billetts in June last year following a disagreement over the direction of the company and have recently completed their period of gardening leave.