Prison inmate Charles Newell, 26, injured critically when he was knocked out in a welterweight boxing match nine days ago, died here today of brain injuries.

He was the third U.S. boxer to die from bout-related injuries since Nov. 28.

Newell, who was serving a six- to 12- year term for robbery at the state prison at Enfield, died at St. Francis Hospital about 2 p.m. without having regained consciousness, a state corrections department spokeswoman said.

Newell underwent surgery within hours after he was taken to the hospital, where doctors removed a blood clot to relieve pressure inside the skull.

Newell was knocked unconscious Jan. 9 by Marlon Starling in the seventh round of a scheduled eight-round fight. It was not clear whether the injury was caused by the punch or the fall.

Renewed debate over boxing safety began after middleweight fighter Willie Classen died Nov. 28, five days after being injured in a bout at Madison Square Garden. Then, 20-year-old Tony Thomas died Jan. 1 of injuries received in a South Carolina fight Dec. 22.

(The New York State Athletic Commission announced yesterday that referee Lew Eskin and Classen's handlers have been suspended for their part in the fight. The suspensions range from six months to a year.)

An investigation into Newell's death is almost completed and will be submitted to Gov. Ella Grasso next week, according to Mary Heslin, state consumer protection commissioner, whose department oversees boxing in the state.

Heslin said that the investigation so far has shown that "everything was in order" during the fight and that she had no immediate plans to suspend boxing in the state.

Heslin said that all precautions recently initiated because of the deaths of Classen and Thomas were followed.

"I don't feel that because of one unfortunate death there should be a panic-button struck," she said, adding that "some subsequent action would be taken by this agency."