Farm will sit within Cello's brand consulting division, which already includes Leith, The Value Engineers and TMI.
Farm and Leith London will join to give Cello more of a force in the London agency market, creating an agency with combined billings of around £42m and 65 staff. Cello hopes the merger will enable it to diversify into digital, brand experience and new media channels.
Rob Smith, managing partner of Farm, said: "The emphasis on data, research and added value thinking within the Cello Group was a major attraction and will be of real value to our clients.
"The whole style of the group, with its partnership structure, means an incredibly good fit with our culture -- we're joining a peer group that has bought into this concept rather than sold out."
Jeremy Pyne, managing partner of Leith London, and Smith will remain in their positions, reporting to Leith chief executive John Rowley, who will become chairman of the new agency.
Throughout the integration process the two agencies will continue to operate under their current names and from their current offices.
Mark Scott, Cello chief executive, said: "The combination of Farm and Leith London will give us critical mass within the London market."
In December last year, Cello acquired Field Management, a data collection and research agency, the third of its research agency acquisitions in 12 months.
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