The unit, which will be call-ed BMBpr, will be housed in the agency's Covent Garden offices.
BMB has yet to officially appoint a PR executive to front the company, but the agency is in advanced talks with a number of candidates and is close to making some appointments.
Initially, BMB planned to make an acquisition in the PR sector in order to bolster its offering but has since decided to grow the business organically, in a bid to retain more control over the unit.
Andrew McGuinness, a founding partner at BMB, said: "At heart, we've always been a publicity company.
There's a real shakedown taking place right now and we're focused on developing our offering to capitalise on it. PR's a natural next step for BMB."
The decision to launch a PR company comes shortly after the marketing services group Cheil Worldwide bought a 49 per cent stake in the agency.