BBC News - JK Rowling 'anger' at legal firm over pseudonym leak: Russells Solicitors said it apologised "unreservedly".
In a statement, it said the wife of one of its partners, Chris Gossage, had told her best friend, Judith Callegari, that Robert Galbraith was really Rowling.
The company said it was revealed "during a private conversation" adding "the disclosure was made in confidence to someone he trusted implicitly".
It has been reported Ms Callegari subsequently revealed Rowling's identity to a Sunday Times journalist via Twitter on 9 July.
Russells continued: "Whilst accepting [Gossage's] own culpability, the disclosure was made in confidence to someone he trusted implicitly. On becoming aware of the circumstances, we immediately notified JK Rowling's agent."