The agency, which won the business following a competitive pitch handled by Creativebrief, will take charge of three brands: London Essence Company, which makes tonics and mixers; botanical sparkling drink Thomas & Evans; and Monte Rosso, a soft drink with flavours inspired by apéritifs such as Aperol.
Creativebrief ran the pitch process on a "try before you buy" basis, Britvic said: following chemistry meetings, Leagas Delaney was given a four-week trial, which led to the long-term appointment after WiseHead Productions regarded it as a success.
The appointment will not affect Saatchi & Saatchi or VCCP, which were given responsibility last year for Robinsons, and for J2O, R Whites and Tango respectively.
Jonathan Gatward, global category and capability director for Britvic, said: "We were impressed with Leagas Delaney’s understanding of our business’ goals, as well as its creative and integrated strategic approach. I am confident in its ability to deliver exceptional results for the WiseHead Productions brands."
Fergus Hay, chief executive of Leagas Delaney, added: "The entrepreneurial and pioneering spirit of Britvic’s WiseHead Productions is one that matched well with our own, while the high quality of the products reflects the business’ commitment to excellence."