The Internal Revenue Service disclosed yesterday that its failure to sign 31 documents could cost $22.6 million in lost taxes.

The agency confirmed a report by a private group, the Coalition of IRS Whistleblowers, that the errors had been made. But the IRS rejected the coalition's estimate that the mistakes involved 1 million tax returns and $13 billion or more in revenue.

In fact, said IRS spokesman Frank Keith, the foulup involved 10,700 tax returns and additional tax assessments totaling $46 million. Since the problem was discovered, he said, the IRS has issued new assessments for $23.4 million in 7,600 of those cases.

Keith said the three-year statute of limitations has expired in the other 3,100 cases, involving a maximum of $22.6 million, much of which already has been paid by taxpayers. Any of that money paid during the last two years will be refunded without further action by the taxpayer, Keith said, adding that the agency has no authority to rebate payments made more than two years ago.

Keith said the problem involves IRS Form 23C, a summary document or cover letter. That form must be signed before additional taxes can be assessed against the tax returns to which it is attached. One form can cover hundreds of thousands of returns and other tax documents.

The IRS discovered recently that 31 of those forms had not been signed over the last five years.

The coalition, affiliated with the Church of Scientology, obtained 25 pages of internal IRS documents, dated April 12, 1990, that explained to field offices how the problem should be handled.

One document noted, "We must ensure that taxpayer contacts are handled timely, consistently and professionally to minimize any loss of confidence by the public."

Keith said that during the five-year period when 31 Forms 23C were not signed, the IRS properly processed 20,000 and assessed additional taxes in 250 million cases.