The move signals an attempt by LU to address more effectively its myriad PR challenges, from communicating security measures to encouraging tourists and commuters to use the world's oldest underground system.
The Tube, often maligned for its creaking infrastructure, was under the global media spotlight in the wake of the 7 July terror attacks.
LU head of communications Andrew Jones said: "A roster will deliver more effective communications than an agency retainer."
Bite Communications has held an account with LU for three years and will be under consideration for a place on the roster.
The search is being conducted through the AAR. It plans to have a shortlist in place by the beginning of November. The roster will be reviewed every two years.
AAR head of business development Nadia Martins said LU expected a "best in class" service from the roster, with agencies providing specialist expertise and the ability to manage projects as they arise.
The roster will include an agency with arts expertise to promote cultural brand features such as Poems on the Underground.
Jones said the roster was unconnected to 7/7. According to Transport for London, passenger numbers -- which fell by as much as 15% on weekdays following the attacks - have returned to their usual levels.
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