Asset manager Affiliated Managers Group has purchased an equity interest in $4.8bn (€3.2bn) credit derivatives firm Blue Mountain Capital Management, its third investment in as many months, as it continues to build its portfolio of boutiques.

Under the terms of the agreement, BlueMountain's management team will maintain a majority stake in their business. Andrew Feldstein, BlueMountain's chief executive, said the pact would allow the company to retain autonomy and put it in a better position for long-term growth.