The contractor will refurbish 20 Moorgate in the City of London to allow staff to move into the offices before the end of 2012.
The office building will used by the Prudential Regulation Authority, the replacement body for the Financial Services Authority.
Prior to buying the new office block, the job was estimated to be worth up to £20m and attracted interest from all the big London fit-out specialists including Como, Kier Wallis, ISG and Overbury.