In April, Credit Suisse's co-head of investment banking, Tim O'Hara, had said the bank is fully committed to the business.

Both Credit Suisse and Deutsche Bank are in the middle of global strategic reviews, spokeswomen for the banks separately said.

The gains at the U.S. banks underscore how being faster to comply with new rules can result in greater revenue gains. Analysts said that even if the prime brokerage business itself is often low margin, it can be a crucial source of stock trading revenue.

Coalition estimates that prime brokerage accounted for 35 percent of all equities revenue in 2014, up from 31 percent the year before and 25 percent in 2009. Morgan Stanley, for example, posted an 28 percent increase in stock trading revenue in the second quarter thanks in part to equity prime brokerage.

For Goldman Sachs, stock trading revenue rose 63 percent in the second quarter from the same period a year earlier, also helped by prime brokerage. Goldman chief financial officer Harvey Schwartz said that the bank has been benefiting from higher pricing, though it was not the primary driver of performance.

For the ten biggest global investment and universal banks, prime brokerage revenue in 2014 grew to about $14.2 billion, according to research firm Coalition, up 11 percent from the year before.

A Reversal

U.S. banks have been able to boost revenue in part by raising prices. One prime brokerage executive at a European bank estimated that financing a stock trade could be as much as five times more expensive than it had been. Many executives across Wall Street stressed that without charging more, the business cannot be profitable for them anymore, because prime brokerage demands so much capital.

Prices for services can vary widely even from one customer to another at the same bank, depending on such things as how big the fund is and how much business it does with the bank.

Many banks are pressing customers to either do more trading with them or find a new prime broker. For hedge funds, that is translating to more of them working with fewer prime brokers: in 2012, 54.6 percent of newly launched hedge funds worked with one prime broker. In 2015, that figure had risen to 71.2 percent, according to industry data tracker Eurekahedge.