It also emerged that the same student had recently been dismissed from an academic job for plagiarising a book review.
The student, Gary Owen Hughes, is now thought to be working for an insurance
The decision by Oxford's Hebdomadal council, the university's executive body, was disclosed after more than two years of assessment by an independent academic.
Mr Hughes was a student of American history and earned his Doctor of Philosophy degree in 1988 after writing a
thesis on the politics of Pennsylvania during the American revolution.
Professor Jack Pole, former Rhodes Professor of American History and Institutions at Oxford, and Mr Hughes's former supervisor, said last night that the thesis had been in part, at least, copied from another unpublished dissertation held in Princeton University Library.