The American Institute of Certified Public Accountants is warning the Internal Revenue Service that its proposed voluntary tax preparer certification program is arbitrary, capricious and illegal.

AICPA Board Chairman William Balhoff accused the program of potentially being an agency end-run around a Supreme Court decision barring IRS regulation of preparers.

While the program is being billed as voluntary, Balhoff said: “In reality, tax return preparers would face an overwhelming, compelled incentive to participate in the IRS’s credentialing program," meaning that the proposed program will be de facto mandatory.

In a Tuesday letter to IRS Commissioner John Koskinen, Balhoff noted the agency already has the means to penalize tax preparers for illegal activities. The IRS can impose civil fines for errors in returns such as reckless understatement of income and failure to sign a return. The IRS also can file criminal charges against someone who submits fraudulent returns, he said.

Balhoff added the IRS can also seek court injunctions against individuals barring them from preparing returns.

About one fifth of AICPA’s 394,000 members globally perform financial planning services.