A Securities and Exchange Commission official admonished accounting firms yesterday for taking lucrative contingent fees from companies whose books they review.

SEC Chief Accountant Donald T. Nicolaisen said the practice jeopardizes auditor independence because it leads accountants and their clients to share financial interests. The nation's biggest accounting firms have reaped millions of dollars in contingent fees for peddling tax shelters and other aggressive tax advice, analysts say.

Nicolaisen's letter came in response to a recent inquiry by the American Institute of Certified Public Accountants, the industry's biggest trade group, which has clashed with regulators in recent years amid high-profile corporate scandals.

Tax work has become increasingly controversial in the accounting industry. While the landmark 2002 Sarbanes-Oxley Act barred accountants from performing technology consulting and several other services for audit clients, it did not prohibit accountants from doing tax work for those clients -- a practice that consumer groups and some shareholder advocates adamantly oppose.

Contingent fees between tax advisers and clients sometimes are based on whether a court or regulator approves a tax strategy, and whether a client company saves a particular amount in the process.

Accounting firms had relied on the fact that fees were subject to approval as proof that there was no conflict. But that is not enough to remove the taint to the accounting firm's independence, Nicolaisen said yesterday. Fees reviewed by judges or federal agencies after a tax victory are only proper if the accounting firm and the client had not previously agreed that the accountants would reap a percentage of the tax savings involved.

Nicolaisen also cautioned that bonuses that clients sometimes offer tax experts at the end of a regulatory win also might run afoul of the rules. "Both the accounting firm's interest in encouraging a large payment and the leverage provided to the client to withhold or reduce such a payment if it is in any way dissatisfied with the firm could have an adverse affect on a reasonable investor's conclusion that the accounting firm is capable of exercising objective and impartial judgment," Nicolaisen wrote.

Robert J. Becton, a spokesman for the AICPA, said the group is still reviewing the letter and its implications. "This is just part of the ongoing dialogue with the agency. . . one of the many issues we go back and forth on." Becton said the SEC had issued some "very clear guidance."

Top accounting industry officials said they have noticed a drop in the number of audit clients who hired the same firm to provide tax advice in the past year.

Several members of the accounting industry oversight board, which took over responsibility from the AICPA for setting professional standards and punishing errant auditors, have expressed interest in reopening debate on what kinds of tax advice auditors may provide, after a series of recent lawsuits and congressional hearings focused more attention on tax shelters.