Credit Suisse and Barclays, two of the biggest operators of dark pools, have entered settlement negotiations with the New York attorney general and the Securities and Exchange Commission over allegations of wrongdoing in the private trading venues, said people familiar with the matter.

Credit Suisse is in talks to pay a fine in the high tens of millions, which would be the largest fine ever levied against an operator of a private trading venue, and the Barclays discussions also suggest a large fine, the people said.