The merger is a result of two years of informal talks between the two shops, which have often pitched against each other, most recently for Butlins. Beechwood won.
Equity is to be divided between five senior managers, who say the merger will combine RPM3's above-the-line capabilities with Beechwood's direct marketing and digital expertise.
Philip Beeching, the Beechwood chairman, will occupy the same role at the combined agency. RPM3's Mark Brandis and Beechwood's John Wood will be managing directors. Martin Butler, the RPM 3 chief executive, will step back from day-to-day involvement in the agency, but will still retain a shareholding.
The 60-strong agency, working under the temporary name of RPM3 Beechwood, will have a client list that includes Brantano footwear, Alliance & Leicester, HMV and Matalan. Neither redundancies nor client conflicts are expected.