An estimated 40,000 people with incomes over $100,000 who fail to file tax returns are less likely to be caught by the IRS than are average earners who don't file, congressional auditors said Tuesday.

The General Accounting Office cited this "ironic imbalance" while reporting on efforts by the Internal Revenue Service to reduce the tax gap - the difference between what taxpayers owe and what they pay voluntarily. The gap was estimated at $85 billion in 1987 and is expected to grow to $114 billion by next year.IRS Commissioner Fred T. Goldberg Jr. said the agency is trying to correct the imbalance.

The GAO report focused on two parts of the 1987 gap: $48 billion lost because of underreported income and $7 billion that was not collected because as many as 6 million people failed to file a return. The report was presented to the House Government Operations subcommittee on consumer affairs.

"High-income non-filers, or those making over $100,000 a year, have a better chance of escaping IRS scrutiny than non-filers with lower income," Paul Posner, an associate director of GAO, told the committee. He cited two reasons:

-The IRS formula for determining which non-filer cases to pursue understates the chances of gaining significant revenue from high-income cases. In one test, GAO found that the average yield from selected high-income cases was $7,811, nearly triple the estimated $2,967.

-The IRS arbitrarily excludes high-income cases from its substitute-return program. This program uses W-2 wage statements and similar earnings statements to estimate the income of a non-filer; the IRS then sends the person a bill based on that estimate. One GAO test showed the program producing $1,700 in taxes for each $1 spent.

The IRS concluded the substitute-return program would understate taxes owed by well-to-do people because large segments of their income are not reflected on such statements.

"We believe that the alternative is worse - high-income non-filers escaping any IRS tax assessment," Posner said.

GAO sampled 300 high-income non-filers and found them to have an average age of 46 and median income of $134,000; 68 percent were married; 55 percent showed wages as their predominant source of income. Eleven percent had income over $300,000.

Even after a high-income non-filer is identified and files a return, GAO concluded, the IRS makes no systematic checks to see if the return properly states income and deductions.

Goldberg said the IRS, in response to the GAO report, already has begun implementing recommended changes.

In the future, he said, high-income non-filer cases that still are not settled after collection actions are taken will be referred to IRS auditors. All such returns that are delinquent will be checked for unreported income and overstated deductions. And the formula for estimating the yield of high-income cases will be revised.

"There is no reason we shouldn't work every one of these high-income non-filers and we are going to," Goldberg told the subcommittee.

Ironically, Goldberg said, as many as 40 percent of Americans who do not file returns as required are entitled to refunds. He blamed that fact on a complex tax system and fear of the IRS.

"It is unfair that so many low-income people are so scared of government" that they don't file a return to qualify for tax credits and receive refunds, he said.

Rep. Doug Barnard, D-Ga., chairman of the subcommittee, said the IRS must be more vigilant in finding high-income individuals who don't pay their share. "It makes that poor Joe Sixpack feel pretty bad" to read about millionaires escaping taxes "when one of your diligent agents is on his back for $600," Barnard said.

"I agree," Goldberg said, noting that the IRS has begun shifting more of its audit and collection efforts toward rich individuals and big corporations.