The decision means agencies such as Olive 360 - the event management consultancy that BSI acquired in 2007 - Abode, IR and ICOR will cease to exist in their present form.
BSI group managing director Trevor Elswood said the amalgamation was in response to feedback from clients, rather than any reaction to market conditions, and he insisted there would be no job losses as a result.
"Our customers have been saying to us that their strength of feeling is towards the BSI brand," he said. "They trust BSI group to look after them and that's where we need to harness our marketing and our people."
He added that the rationalisation, which takes effect on 1 November, will see a total of around 75 meetings and events staff working under one umbrella.
Elswood said that also though the group had been affected by cutbacks by existing clients, a number of new wins this year, including Tesco, E-on, Cable & Wireless and Dixons Group, had redressed the balance and made 2009 a positive year.