The successful agency will be required to provide a full digital service over the next three years.
Publicis Modem will retain responsibility for the creative digital element of the British Army's recruitment marketing.
The overall aim of the army's recruitment marketing strategy is to inspire and raise awareness of army careers, which have received a recessional boost as higher quality candidates consider military careers as jobs in the private sector dry up.
The digital element of the strategy will complement and support all recruitment marketing, covering both regional and national campaigns.
A selection of agencies have been asked to pitch for the work.
Michael Smith, deputy director of Interactive Services at COI, said: "Digital communications have become a key component of the army's recruiting strategy.
"COI is committed to helping the army find the agency that most closely matches its technical digital needs at this time."
The British Army launched the 'Start Thinking Soldier' campaign in April. The TV and online campaign encourages potential recruits to think about how they would tackle the type of challenges faced by soldiers.
Four different executions show a different mission from the first person perspective of an army recruit.
At the end of each ad the viewer is directed to armyjobs.mod.uk where they can decide how they would conclude the mission. They are then given immediate feedback on their choice.
Once online they can also take part in a range of challenges covering observation, problem solving, intelligence, memory, navigation and awareness.
Publicis created the TV ads, while its digital division Publicis Modem created the online elements.
Colin Cook, marketing director of Army Recruitment Group, said: "The success of our Start Thinking Soldier campaign has demonstrated the potential of innovative and effective digital marketing within army recruitment.
"We are therefore delighted to confirm Publicis Modem as our digital advertising agency and look forward to appointing a technical digital agency to bolster our digital capacity still further."