One of the country’s most influential researchers in cancer screening has resigned from his post at Dartmouth College, after a two-year internal investigation concluded he had plagiarized a graph included in a paper published in a prominent journal.

The researcher, Dr. H. Gilbert Welch, has published widely on the risks of aggressive screening and over-diagnosis, including Op-Ed articles in The Times and several popular books. He disputed the university’s findings against him.

“I am saddened to say that I am resigning from Dartmouth,” Dr. Welch wrote in an email to colleagues. “I feel that I can no longer participate in the research misconduct process against me — as I fear my participation only serves to validate it.”

In a prepared statement, Dartmouth said the university had “reviewed this matter in accordance with its research misconduct policy and procedures, which defines plagiarism as ‘the appropriation of another person’s ideas, processes, results or words without giving them appropriate credit,’” and found Dr. Welch guilty.