It said that while Gossage was culpable, "the disclosure was made in confidence to someone he trusted implicitly."
Russells, a specialist in entertainment law, said it had informed Rowling and her agent once it learned what had happened.
"We can confirm that this leak was not part of any marketing plan and that neither J.K. Rowling, her agent nor publishers were in any way involved," the statement said.
The Cuckoo's Calling had garnered good reviews but sold in the hundreds of copies since being published in April. Since Rowling was outed as the author, it has topped best-seller lists, with publisher Little, Brown and Company commissioning a new printing of 300,000 copies.
Rowling said that "only a tiny number of people knew my pseudonym and it has not been pleasant to wonder for days how a woman whom I had never heard of prior to Sunday night could have found out something that many of my oldest friends did not know."
"To say that I am disappointed is an understatement.
"I had assumed that I could expect total confidentiality from Russells, a reputable professional firm, and I feel very angry that my trust turned out to be misplaced."
- AP
