Allen & Overy's (A&O) multimillion-pound Spitalfields headquarters is under threat from the controversial CrossRail project.
The magic circle firm is among several hundred petitioners who are in ongoing discussions with the Secretary of State for Transport regarding the CrossRail bill.
The firm does not object to the proposed railway, but in its petition A&O says that although the railway will pass its new building below ground level, it was concerned about the powers contained in the bill that allows CrossRail to acquire part of the basement. A&O plans to use the basement for an auditorium, client meeting rooms and a car park.
An A&O spokesman said: "We continue to work with CrossRail with this and don't anticipate any of the building will be affected by the project."
A&O is expected to move into its new buiding at 1-10 Bishops Square in the autumn. The firm is planning to sublet up to one-third of the building, but has not yet secured sub-tenants.
The firm's limited-liability partnership accounts say it had set aside £42m to fund the building's fitout and has entered into separate banking facilities of up to £50m to finance the rest of the move.