The International Boxing Federation on Friday bowed to pressure of a fast-approaching $35 million extravaganza and agreed to sanction a 12-round heavyweight title fight between Mike Tyson and Michael Spinks.

The decision ended weeks of haggling among the IBF, the rival World Boxing Council and the World Boxing Association over whether the bout Monday night would be 12 or 15 rounds. It also guaranteed that there will be an undisputed champion following the fight.The IBF had threatened to strip Tyson of its title if he fought anything but a 15-round fight against Spinks.

However, the threat was withdrawn when the IBF reversed an earlier decision and granted Tyson an exception to its Rule 21, which calls for all title fights to be 15 rounds in length, IBF President Bob Lee said.

Lee said the exception was passed unanimously late Thursday and Friday by the IBF's championship committee and its executive committee. The vote also ended a federal lawsuit the IBF had brought against the WBA, the WBC, state athletic commissioner Larry Hazzard and Bill Cayton, Tyson's manager.

"I think we agreed to 12 rounds because of the best interests of the public and because they think this is for the undisputed heavyweight championship," Lee said after relaying the IBF's decision to U.S. District Court Judge Alfred M. Wolin. "We didn't do it in thebest interests of boxing."

Cayton, the WBC and the WBA had called for a 12-round fight all along, citing safety issues.

"I think the IBF, when the time arrived, made a a very wise decision," Cayton said in a telephone interview from Atlantic City. "Much of the credit has to go to Larry Hazzard, along with the judge."

The dispute between the organizations centered on a July 1987 agreement between the IBF, the WBC and the WBA to rotate as the lead organization for Tyson's defenses of his title.

The IBF was the lead organization for the Tyson-Tyrell Biggs fight, the WBC handled the Tyson-Larry Holmes fight and the WBA was in charge of the Tyson-Tony Tubbs match.

The IBF is the lead organization for the Tyson-Spinks fight, and as such it maintained that its championship rules should be in effect, meaning 15 rounds.

-- Publicity over the personal problems surrounding Mike Tyson has resulted in a shift in betting odds, but the heavyweight champion remains a solid 4-1 favorite in Las Vegas to beat Michael Spinks in their title fight Monday night.

Odds on Tyson had been as high as 5-1 this month, according to Nate Wolfson, manager of the sports book at Bally's Hotel and Casino.