WASHINGTON — Intuit Inc., H&R Block Inc., and 18 other companies that sell tax-return preparation services have agreed to offer free electronic filing to as many as 93 million taxpayers in a four-year agreement that keeps the IRS out of the tax software market.

The agreement makes taxpayers with adjusted gross incomes of less than $49,600 eligible for free filing services from the companies, which qualifies about 70 percent of the taxpaying public. About 5.1 million tax returns were filed for free under the program last year.

Intuit has sought the agreement in part because it places limits on the strategy of some rivals of offering free filing to all customers while profiting from selling short-term tax refund loans with high interest rates and other auxiliary products. The agreement also prevents the IRS from developing its own free tax- filing systems and making them available to the public.

The Internal Revenue Service said the agreement would help the agency meet a congressional mandate to have 80 percent of all tax returns filed electronically by 2007. It would also impose new consumer protections for taxpayers enticed by the short-term refund loans that carry high fees, IRS Commissioner Mark Everson said in a statement.

"We think this is a constructive step forward," Everson said. "It allows free file to continue to grow but it also contains important new consumer protections."

The companies ended their negotiations with the tax collector two days after Mountain View, Calif.-based Intuit, the world's biggest maker of tax-preparation software, won a legislative victory when the U.S. Senate voted to ban the IRS from developing its own products to help taxpayers file electronic returns.

While the IRS hasn't begun developing such software, Wall Street analysts said the threat that it might change its mind gave the government leverage in the negotiations to persuade companies to provide more services.

Tim Hugo, executive director of the Free Filing Alliance, a coalition of tax-preparation service companies, said the agreement confirms that the private sector is best positioned to boost interest in electronic tax filing.

"The Free File Alliance has made a huge contribution to the dramatic growth in e-filing over the last three years," said Hugo, a Virginia legislator.

Treasury Department spokes-man Taylor Griffin said the administration "is committed to work with the private sector to provide free filing."