The head of the Internal Revenue Service faces questions in Congress today about auditors’ complaints that they are being forced to close corporate cases prematurely, allowing billions in tax dollars to go unpaid.

In interviews, these revenue agents warned that unless they were free to pursue what their instincts tell them, their focus would end up being only on known abuses, and new ones created by the tax advice industry would go undetected.

The agency countered that it had increased the number of companies whose tax returns it examined by a fourth since 2001, even though the number of auditors was virtually the same.

Agency officials said this was accomplished by cutting back slightly on audits of the very largest companies, which produce more than 80 percent of all corporate profits, while increasing audits of those with assets of $10 million to $250 million. At the same time, the officials say, they have shortened the average time to complete an audit from almost two years in 2001 to less than 18 months last year.