WASHINGTON, Dec. 6 - Medicare beneficiaries will not be allowed to buy insurance to cover their share of prescription drug costs under the new Medicare bill to be signed on Monday by President Bush, the legislation says.

Millions of Medicare beneficiaries have bought private insurance to fill gaps in Medicare. But a little-noticed provision of the legislation prohibits the sale of any Medigap policy that would help pay drug costs after Jan. 1, 2006, when the new Medicare drug benefit becomes available.

This is one of many surprises awaiting beneficiaries, who will find big gaps in the drug benefit and might want private insurance to plug the holes - just as they buy insurance to supplement Medicare coverage of doctors’ services and hospital care.

Congress cited two reasons for banning the sale of Medigap drug policies. Lawmakers wanted to prevent duplication of the new Medicare benefit. They also wanted to be sure that beneficiaries would bear some of the cost. Health economists have long asserted that when beneficiaries are insulated from the costs, they tend to overuse medical services.

The new drug benefit would be the biggest expansion of Medicare since creation of the program in 1965. But patients would still face substantial costs.

Under the standard Medicare drug benefit, the beneficiary would be responsible for a $250 deductible, 25 percent of drug costs from $251 to $2,250 and all of the next $2,850 in drug costs. Private Medigap policies could not cover any of those costs.