Transport for London (TfL) and its group company London Underground Ltd (LUL) are reviewing their panel.
The new arrangements will be in place by the summer and will replace LUL's existing panel arrangements, which expire in July this year.
TfL director of legal and compliance Gareth John said: "This is an opportunity to clarify our expectations, to ensure that we have the right mix of expertise and resource, and to drive best value and efficiencies."
Invitations to tender have been sent to the 18 firms that have been selected to take part in the process. These are likely to include present advisers Berwin Leighton Paisner, Bird & Bird, Hammonds, Herbert Smith, K&L Gates and Manches.
TfL's arrangement with Eversheds for mainstream operational work will not be affected by the review.
TfL's investment programme includes the extension of the East London Line, the transfer of the North London Railway, the new Thames Gateway Bridge and the extension of the Congestion Charge zone, as well as commercial property developments and a plan to raise finance on the capital markets.
Firms will also be required to manage the legal aspects of LUL's portfolio of PFI contracts with Tube Lines and Metronet.
LUL legal director Sarah Atkins said: "We're at defining moments in many of our key contractual relationships and in situations with no obvious precedents. We therefore need advisers who can think laterally, but who also have the pragmatism needed to support the successful delivery of construction projects."