WASHINGTON — The Internal Revenue Service said today it is prepared to offer settlements to some 45 corporations if they agree to end questionable tax shelter practices.

The IRS Commissioner, Doug Shulman, said the agency aims to end transactions that have allowed corporations, including many of the nation’s top banks, to gain billions in tax deferrals. He did not name the corporations.

Letters were sent out today giving the corporations 30 days to make a decision on accepting the terms of the offer, which include best efforts to terminate the transactions by the end of this year.

Mr. Shulman said the transactions, called lease-in/lease-out (LILO), and sale-in/lease-out (SILO), involve complex arrangements where large corporations lease or purchase large assets such as foreign rail systems or sewer systems and then lease them back to the original owner.

“As a basic matter of fairness to all taxpayers, the IRS cannot allow LILO and SILO deals to stand,” he said.

The 45 companies are engaged in more than 1,000 of these LILO and SILO transactions. The tax agency said it has won three cases against corporations involved in the transactions, giving others an incentive to agree to the settlement offer.

It said that cases will proceed against corporations that decide not to participate in the settlement.