WASHINGTON -- Congress is poised to ban horse slaughter in the United States after lawmakers agreed Wednesday to stop paying for federal inspectors at horse meatpacking plants.

Lawmakers on a Senate-House committee approved the ban but allowed the slaughterhouses to continue operations for 120 days after the bill is signed into the law, according to Senate aides and horse advocacy groups familiar with the provision.

The measure was included in a spending bill for the Agriculture Department. Both the House and the Senate had added the ban into the bill, but some lawmakers had threatened to remove the measure in final negotiations. The final bill must now be approved by Congress.

"This prohibits the American horse from being killed in the United States to become food for foreign dinner tables," said Michael Markarian, executive vice president of the Humane Society of the United States.

Markarian said an estimated 100,000 horses would be saved next year from slaughter in the United States, Canada and Mexico. The provision also bans the transportation of horses to slaughter across the border.

The United States is home to three slaughter plants, two in Texas and one in Illinois. Jim Bradshaw, a consultant for the two Texas facilities -- Dallas Crown Inc. of Kaufman and Beltex Corp. of Fort Worth -- said Wednesday the law could put them out of business, costing 150 jobs.

"We don't have a clear answer about what's going to happen," said Bradshaw, who noted the plants needs federal inspectors to legally operate.

The three plants slaughter and package horse meat to buyers in Japan, France, Belgium, Italy, Germany, Mexico and Canada.

Chris Heyde, a policy analyst for the Society for Animal Protective Legislation, said he was nervous about the four-month delay written into the new law.

"I question the 120 day delay I think they've got something up their sleeve on this," Heyde said.

Senate aides said the delay was proposed as compromise to keep the slaughter ban intact. Because the ban is in an annual spending bill it is only be valid until Sept. 30, 2006, the end of the federal fiscal year.

In the long-run, horse advocacy groups have pressed for a more permanent approach making the transportation of horses for slaughter a violation of interstate commerce law in the Constitution. Sen. John Ensign, R-Nev., introduced a bill to do that earlier this week.

In a speech last month, Ensign, described horses as "an American icon" that should be saved from foreign menus. Ensign, a veterinarian, said old and ailing horses should be euthanized in a humane way.