Teamster President Jackie Presser has been diagnosed as having brain cancer, touching off a power struggle over who will run the 1.7 million member union, sources inside the organization said Saturday.

A special meeting of the Teamster executive board was called for Monday in Scottsdale, Ariz., near Phoenix, where Presser is undergoing tests to determine whether it is feasible to operate on the up to four brain tumors that have been discovered, a union source said.Presser's deteriorating condition appears to assure that his trial on federal labor fraud charges, previously postponed to July 12 in Cleveland, now will not take place.

On May 4, Presser, 61, then at the Cleveland clinic undergoing tests and radiation therapy, told the executive board that he was taking a four-month leave of absence because clinic doctors had advised him to avoid all forms of stress during the radiation treatment. Presser had a malignant tumor removed from his lung in January 1987, and then underwent radiation treatments that doctors said left him in a weakened condition and mentally confused.

But the doctors said there had been no recurrence of cancer.

Communicating with the board via an internal union computer hookup, Presser said Mathis would serve as president during his leave of absence as provided by the Teamsters' constitution.

On Thursday, however, Presser checked into the Barrows Neurological Institute of St. Joseph's Hospital in Phoenix, after the brain tumors were discovered. Officials at the institute declined Saturday to release any information on Presser or to relay calls to his doctors there.

Mathis called the special meeting of the executive board to tell the other 16 members of the board of Presser's condition. But then at least six international vice presidents on the board led by Joseph Trerotola sent word that they would boycott the session, although they initially had urged that the meeting be held.

None of the six could be reached Saturday, and the purpose of the anticipated boycott could not be learned. Under the union's constitution, Mathis as secretary-treasurer fills in for Presser if he is temporarily unavailable for up to six months. After that period, the board members convene to decide if the disability is permanent and, if so, to choose a successor from among themselves.