Porter Novelli, Four Communications, Regester Larkin and Proof Communications have all been engaged.
They will cover a broad spectrum of activity including arts, business, science and technology, digital, policy and crisis comms on a project basis.
British Library head of communications Gill Webber said: 'The British Library communications team covers a vast brief, dealing with everything from acquisitions and award-winning exhibitions, to our work on Digital Britain and the copyright debate.'
The British Library is the national library of the UK and one of the world's leading research libraries. It provides information services to the academic, business, research and scientific communities.