Some of the nation's biggest companies face a deadline today for completing reviews of their internal financial controls, a labor-intensive, costly effort that has created intense friction between corporate managers and auditors.

Top regulatory officials repeatedly have warned that a significant minority of companies, from a few hundred to a thousand, may report serious weaknesses in their fiscal checks and balances, which could have repercussions in the price of stock shares.

Controls are the backbone of a company's finance system. They include such things as whether multiple officials are required to sign off on company checks and whether employee expense reports are scrutinized by managers.

Corporate scandals of the past several years prompted Congress to require the reviews in hopes of preventing future fraud.

Major companies with fiscal years ending today are supposed to have completed their reviews, imposed under the 2002 Sarbanes-Oxley Act, by now -- the first in a series of deadlines. Other large firms will face deadlines as their fiscal years come to an end, rolling through the next 12 months. Smaller and foreign companies are required to finish documenting controls by July 15.

The law requires chief executives and finance chiefs to attest to the strength of their financial controls, and it compels auditors to review the way companies document their systems.

PricewaterhouseCoopers LLP, the largest accounting firm in the country, informally surveyed 700 clients, estimating that 10 percent of companies are at "severe risk" of not finishing the work on time and that another 20 percent might soon slip into that category, according to a speech last week by chief executive Dennis M. Nally.

"There's no question that [the control review] presents daunting challenges," Nally said.

Chief among them, corporate trade groups say, is the cost. Overall, the expense of complying with the Sarbanes-Oxley Act has reached $5.1 million for the average U.S. company, according to a study by executive recruiting firm Korn/Ferry International. That is a $2 million increase from an estimate released in July by a financial executives trade group.

For the largest companies, reviewing and documenting financial controls has cost far more. General Electric Co., which voluntarily adopted the process last year, spent $30 million on its effort. It expects to spend somewhat less this year, a spokesman said.

The biggest beneficiaries of the new rules are accounting firms, which will reap increases of as much as a 50 percent in fees for all of the internal control work, accounting experts said.

But working under such tight deadlines and enormous pressure has sometimes soured relationships between auditors and their clients. Chief financial officers and other corporate managers report widespread complaints about resource-strapped auditors who may not have enough time to get the work done, or who may be altering the kind of information they seek from clients in midstream, said Dennis R. Beresford, a University of Georgia accounting professor.

Beresford said it would not surprise him if the number of auditors fired next year increases because of the tensions induced by the control work.

Meanwhile, regulators and investor groups are rushing to educate the public about the issues, so that the stock market does not overreact when large numbers of companies begin disclosing problems in vouching for their controls. Last month, 63 companies publicly revealed problems with financial controls, according to the newsletter Compliance Week. That number is expected to increase over the next several weeks as companies voluntarily notify investors and the Securities and Exchange Commission of problems.

SEC officials emphasize that disclosing problems with controls may mean that companies simply failed to fully document their procedures -- not necessarily that firms have inaccurately reported earnings or other key financial measures.

"The ultimate goal of the . . . requirements is to make sure that the right information is getting into the financial statements," SEC Commissioner Cynthia A. Glassman said.

Accounting experts say that financial control breakdowns can signal broader troubles for a company or its management team. In recent years, the SEC has charged Rite Aid Corp., WorldCom Inc., and Xerox Corp. with having faulty controls, after multimillion-dollar accounting blowups hit all three companies.

Donald T. Nicolaisen, chief accountant at the SEC, told The Washington Post earlier this month that the agency would not grant a broad-based reprieve from the deadline for large companies. But, he said, the agency would monitor the situation closely in the weeks to come and that it might give smaller firms, with fewer resources, a short break from the deadline. Such a decision may not come until after Thanksgiving at the earliest.