It has long been theorized that William Shakespeare was not the sole author of every play that has historically been credited to him. Now a major publisher is making it official. 

Oxford University Press announced Monday it is crediting Shakespeare's British contemporary Christopher Marlowe as a co-author on Shakespeare's "Henry VI" plays in future publications. The decision is the biggest win to date for scholars who have been researching the possibility that Marlowe was the real writer behind many of Shakespeare's biggest works, which has until recently been treated as a mere conspiracy theory. 

The new Oxford edition of the three "Henry VI" plays Shakespeare wrote will be available in November. The volume was edited by four Shakespeare scholars; Gary Taylor of Florida State University, John Jowett of the University of Birmingham, Terri Bourus of Indiana University and Gabriel Egan of De Montfort University. The group reached the decision to credit Marlowe after examining multiple peer-reviewed studies researching the theory. Those studies analyzed variables like how the word choice and syntax in the "Henry VI" plays compared to plays known to have been written by Marlowe. 

Shakespeare Oxford University Press now credits Christopher Marlowe as a co-author on Shakespeare's "Henry VI" plays. Photo: Getty Images

Christopher Marlowe was a playwright previously best known for his works "Hero and Leander," "Edward the Second" and "The Tragical History of Doctor Faustus." Marlowe died at just 29 in a bar fight in 1593. Conspiracy theorists maintain his death was faked so he wouldn’t go to prison for being an atheist. Marlowe was the best known Elizabethan tragedy writer of his time before his death, after which Shakespeare rose to prominence. 

Columbia University professor James Shapiro wrote in his 2005 book, "A Year in the Life of William Shakespeare: 1599," that it was impossible for Marlowe to have been behind any of Shakespeare's plays. 

"I thought I did a damned good job showing that it could only have been Shakespeare who wrote the plays we attributed to him," Shapiro told CNN in 2010. "And I naively thought, that will slow people down who think that Shakespeare didn't write Shakespeare. And they kind of stepped around it."

The controversy over the true authorship of Shakespeare's plays was the subject of Roland Emmerich’s 2011 movie, "Anonymous." As far as Oxford is concerned, there was more truth to the film than viewers originally thought.