Lawyers for JPMorgan Chase have demanded that Bloomberg hand over five years' worth of employee logs, as the bank considers whether to take legal action against the news and data group.
JPMorgan wants to know which Bloomberg employees accessed data on how the bank's staff used its financial terminals.
The demand marks an escalation of pressure on the private company after it emerged that Bloomberg journalists could track how clients used a service that is almost ubiquitous on Wall Street.
"Our legal department sent a formal request to Bloomberg to verify exactly what information reporters had access to and confirmation of their controls to prevent future breaches," said JPMorgan.
People at the bank said it had demanded "logs of all Bloomberg staff who have looked up JPMorgan users and usage since the beginning of 2008 and the role of people who ran each query".
The bank's lawyers also demanded verification that Bloomberg had revoked reporters' access to information about terminal users, as Bloomberg has said it did after a formal complaint from Goldman Sachs last month. They also demanded access to policies and procedures around access to information.
Bloomberg declined to comment.
Other banks said they were watching JPMorgan's move closely but no other client has so far taken such tough steps. In recent days, banks and regulators have expressed concern that Bloomberg journalists were accessing information, such as whether a certain user was logged in, to inform their reporting.
However, while the Bank of England has condemned Bloomberg's actions as "reprehensible", Goldman sounded a more conciliatory tone on Wednesday. "We don't have a major concern," Gary Cohn, its chief operating officer, told CNBC.
Mr Cohn said Dan Doctoroff, Bloomberg's chief executive, "understands the magnitude of the problem" and was reacting positively to address its concerns. Goldman intended to give Bloomberg time to respond once they have "real data" on the issue, he added.
Bloomberg has apologised for allowing reporters access to "limited client information" and appointed an executive to review data compliance policies. It has disabled reporters' access to information about when customers log in, which of the terminals' roughly 15,000 functions they access and what contact they have with its help desk.
People at JPMorgan suspect Bloomberg reporters used login information to try to determine whether Bruno Iksil, the London-based derivatives trader known as the "London Whale", had left the bank.
Bloomberg has emphasised that reporters were never able to see market-sensitive details of "securities-level data, position data [or] trading data", adding that most information gleaned would have been "mundane".
In a memo to clients and staff on Friday, Mr Doctoroff said: "Client trust is our highest priority and the cornerstone of our business."