"Naked" access, a controversial trading practice largely employed by high-speed traders, accounts for nearly 40% of U.S. stock-trading volume, a study has found.

The finding comes amid increasing regulatory concern about the practice, which leaves exchanges in the dark about the identity of the firms doing the trading. That reduces accountability in case of a destabilizing or problematic trade.

While it has been well known that many high-frequency firms used naked access -- trading directly on exchanges using a brokerage's computer identification code -- it hasn't been clear how widespread the practice is. The finding that naked access accounts for such a large part of the market may place more pressure on regulators to increase their scrutiny of the practice.

The report by Aite Group, a Boston research outfit that tracks high-frequency trading, found that naked access accounts for an estimated 38% of the U.S. stock market's average daily trading volume. The firm surveyed and consulted with about 25 major stock exchanges, brokerages, high-frequency traders and companies that cater to high-frequency firms.

Naked access is one form of a more widespread practice called sponsored access in which brokers let trading firms operate on exchanges using the brokers' market participant identification codes. Overall, sponsored access accounts for half of the market's volume, according to the Aite report, which is likely to be released Monday.

Earlier this month, the Securities and Exchange Commission said it would start considering proposals to regulate sponsored access in January. An SEC spokesman declined to comment on the Aite report.

Broadly, there are two ways that a firm can trade on an exchange. It can become a registered broker with the SEC and become a member of an exchange, both of which are costly. Alternatively, a firm can pay a registered broker to use the broker's computer code to trade, or sponsored access.

Some sponsored firms trade through the broker's computer system, giving the broker the ability to employ pretrade checks that could catch risky trades before they reach the market. Traders using naked access, however, trade directly on the exchange and aren't subject to a third party's pretrade checks.

Behind regulators' concerns are the increasingly fast speeds employed by high-frequency traders. According to the Aite report, a firm that uses naked access can execute a trade in 250 to 350 microseconds, compared with 550 to 750 microseconds for trades that travel through a broker's computer system by sponsored access. A microsecond is one-millionth of a second.

The small sliver of time can mean the difference between success and failure in the computer-driven universe of high-frequency trading. It highlights the mind-bending speeds these firms compete at as electronic markets race to provide superfast access.

Critics say naked access heightens the risk of reckless trades that could destabilize the broader market. Exchanges often don't know the identity of firms using sponsored access, since the only way to identify the firms is through the computer code.

That means it could be difficult to quickly track down a firm whose trades have run amok. But exchanges have rushed to offer the service because it brings in huge trading volumes and fees.

"Everyone agrees that some sort of regulatory restriction will happen," says Sang Lee, co-founder of Aite and author of the report. "But it's not as straightforward as most people think."

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