The departure of Southwell, a former journalist who has left to write his sixth book, leaves the lobby body planning to draft in a freelance PRO to help handle media calls in the busy general election period.
Kate Ison, press officer in the BRC's five-strong comms and public affairs team, said: 'We are looking at how we replace David - the set-up of the team is being reviewed.'
A press officer is likely to be recruited on an interim basis, added Ison.
Southwell, a former Sun sub-editor and news editor within the Yellow Advertising Group in Essex, spent two periods at the BRC.
He was initially brought in by former director of comms Ann Grain (now at the RSPCA) six years ago to help handle the BRC's fight back during the 'rip-off Britain' attacks on retailers; after two years, he left in 2000 to write books and freelance.
He was lured back to the BRC in 2001 and became comms chief in 2003 when David Smith quit to join the Energy Networks Association (PRWeek, 23 July 2003).
Southwell described his forthcoming book, which is called Secrets and Lies, as a true tale of the machinations of political parties and corporate business. It will appear this autumn.
Southwell has already penned tomes with titles such as Conspiracy Theories and Dirty Cash.
In addition to Ison, the BRC's comms and PA team also includes public affairs director Sarah Winterton, PA manager James Tyrell and an admin assistant.
BRC members include large multiples, independent retailers and charity shops.