WASHINGTON — A controversial change in accounting rules earlier this year has allowed banks to claim billions of dollars in additional earnings simply by tweaking their bookkeeping, greatly enhancing the appearance that the industry is returning to health.

A study by an accounting expert found that 45 financial firms reported higher first-quarter earnings because of the change. The total benefit exceeded $3 billion. Some large firms, including Prudential Financial and Bank of New York Mellon, were able to report profits rather than losses.

But accounting rulemakers are considering further changes that could drain the blood right back out of the industry, potentially forcing banks to acknowledge paper losses even larger than the new windfall of paper gains.

The proposal has put a spotlight on the Financial Accounting Standards Board, the obscure nonprofit that sets bookkeeping rules for U.S. companies, highlighting the extent to which important changes in financial regulations may be decided off Capitol Hill.

It also has sparked the latest round in a long-running battle between investors eager for more information about the financial health of banks and companies eager to retain control over their image and the information they present to investors.

The proposal, which the standards board will consider issuing for comment later this month, would require banks to report the value of all loans and other assets based on the prices that buyers are willing to pay. This process is called marking to market, and the result is called a fair value. At present, banks are not required to report the fair value of most loans. They can instead report a value based on the original purchase price.

Some investors and accounting experts think the change would make it harder for banks to conceal problems from investors.

"Usually when someone says that something is a big deal in accounting, it's because it's going to increase profits or decrease profits, and this is not going to do either. But this is still a big deal," said Jack Ciesielski, publisher of The Analyst's Accounting Observer. "It's going to show investors how much assets are worth."

Banking executives and industry groups, however, have responded to the proposal with shock and consternation. They say that banks would be forced to record large declines in the value of loans, then set aside large sums to cover the implied losses, tying up money that might otherwise support new lending.

Companies were allowed to adopt the rule in the first quarter and required to adopt it in the second quarter. A study by Ciesielski found that 45 financial firms took advantage of the new rules in the first quarter to report higher earnings. He estimated that the total benefit exceeded $3 billion.

Bank of New York Mellon reported the largest benefit, more than $838 million, a key factor in its quarterly profit of $369 million. A spokesman said that the new rules better reflected the true value of the bank's holdings.

The impact on second-quarter earnings remains unclear. While banks have reported their bottom-line profits and provided investors with some details, they have several weeks to file more detailed disclosures. Wells Fargo already reported a second-quarter benefit of about $664 million.

The new proposal actually applies the same idea to long-term investments, but with dramatically different consequences.

Banks already were required to report fair values for short-term investments. The change this spring made doing so less expensive. But while the new fair value rules are less onerous than the old ones, neither set applies to long-term investments, which make up the vast majority of bank assets.

Even as market prices plunged during the financial crisis, banks reported that most long-term investments had held their value. By requiring banks to report fair values for their entire portfolios, the change could force many banks to acknowledge steep drops in value.