After a keenly fought competitive pitch, the engineering and construction giant has opted to terminate its contract with Weber Shandwick.
Balfour Beatty has used the agency to handle public affairs for more than four years.
The account is understood to be worth significantly more than £100,000 annually in fees and the loss is a fresh blow to Weber Shandwick, following the departure of the agency's highly regarded corporate and public affairs director Michael Prescott.
The former Sunday Times political editor quit Weber Shandwick in January to become group director of comms at BT.
Sources close to the pitch process said Prescott's departure was a key reason for Balfour Beatty parting company with Weber Shandwick.
Balfour Beatty is the largest fixed-rail infrastructure contractor in the world and is working on a number of major UK projects including the M25, Blackfriars Station and the London 2012 Olympics aquatics centre.
The firm's lobbying efforts are overseen by company secretary Chris Vaughan and director of corporate comms Duncan Murray.
PRWeek revealed last month (21 May) that Balfour Beatty was poised to review its UK public affairs requirements.
Weber Shandwick's public affairs chairman Jon McLeod declined to comment.