Underwired will handle electronic customer relationship management for all of Kitcatt Nohr's clients, which include Virgin Holidays, Friends Reunited and VSO.
Underwired will continue to provide online marketing and advertising services to its own clients, such as Penguin Books and Harrods.
Two of the Kitcatt Nohr founders -- Marc Nohr and Jeremy Shaw -- will join Underwired's board as non-executive directors.
The existing board members are Felix Velarde, Underwired's managing partner, and Jason Holland, the creative partner. Lazar Dzamic, the director of planning, who joined from EHS Brann last year, has also been promoted to the board.
Underwired will move into Kitcatt Nohr's offices.
The deal also gives Kitcatt Nohr the option to increase its stake in Underwired at a later date. However, Holland and Velarde remain the majority shareholders in the agency.
Nohr said: "As the internet and other digital platforms gain greater maturity, we find clients increasingly recognise the benefits of planning a strong digital element into their communications."
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