A London firm has ‘apologised unreservedly’ for accidentally revealing that Harry Potter author JK Rowling (pictured) was writing under a pseudonym.

Russells Solicitors, which describes itself as ‘one of the leading firms in the entertainment industry’, has admitted it was the source of the leaked information about the true identity of ‘Robert Galbraith’.

Rowling was revealed as the writer of crime novel The Cuckoo’s Calling in the Sunday Times. She said she had hoped to keep the secret for longer.

In a statement, Russells said the disclosure was caused by one of its partners, Chris Gossage, revealing Galbraith’s true identity to his wife’s best friend, Judith Callegari, during a private conversation.

The statement added: ‘While accepting his 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.

‘We can confirm that this leak was not part of any marketing plan and that neither JK Rowling, her agent nor publishers were in any way involved.’

Russells, established in Soho in 1974, provides legal services to clients from the world of music, theatre, film, television and sport.

On its website, the firm states that it always maintains ‘the highest standard of professionalism and understands the vagaries and idiosyncrasies of the industries and markets in which we operate’.