The agency won the two accounts following a competitive pitch, run by the AAR, against Media-edge:cia and two other agencies.
Mediaedge:cia retains the Warners and Butlins holiday brands, which were not included in the pitch process.
According to Nielsen Media Research, Haven Holidays and British Holidays spend £5 million above the line. The brands spend a further £3 million on direct marketing.
Tim Gibson, the sales and marketing director at Bourne Leisure, said: "We chose BLM because it offered us the best all-round proposition in terms of strategy, planning and buying and expert evaluation."
Earlier this month, Bourne Leisure appointed HDM to handle its through-the-line account following a pitch against Joshua, Quiet Storm and Mustoes.
It was previously held by Publicis.
Haven Holidays is planning a TV campaign starting in December.