One of history's most famous battles could be rewritten by long-lost papal letter

Professor Tom Licence was examining historical records when he came across previously overlooked excerpts from a lost papal letter that was preserved in the Vatican archives, he said.

The letter suggests Pope Alexander II may have backed William the Conqueror's invasion of England because the English had sided with his rival - offering a new explanation for the events that led to the Battle of Hastings.

In the letter, Alexander described England as being led astray by an "evil head" and used striking religious language that Licence said branded the English as "followers of Satan."

It has not typically been viewed as part of a broader political and religious struggle involving the papacy, but Licence said the events of 1066 may have been closely tied to a bitter power struggle between Pope Alexander II and his rival, the antipope Honorius II.

Licence's research suggests England may have sided with Honorius, potentially giving Alexander another reason to support the Norman invasion.

Licence told Fox News Digital he wasn't personally surprised by the connection he made.

"Which papal faction England was aligning with at any given time is always in the back of my mind," the professor said.

Honorius II may not be a familiar name to many - but Licence said he remained a "serious contender" in the papal power struggle, even after his position weakened in 1064.

"In fact, it looked like [Honorius II] was going to make a comeback in 1065," he said.

"Read in that context, the letter from Pope Alexander about England being led astray under an 'evil head' made sense at last. It was never clear to historians in the past what Alexander was referring to."

Licence said he was "confident" the English moved away from backing Alexander II, which would have amounted to "a withdrawal of support."

"It's less clear to me that England openly declared support for Honorius," he said.

"Alexander's rhetoric about the English being led from the path of truth by an 'evil head' - and calling them 'followers of Satan' - suggests, at the very least, that they were leaning that way."

He said that specializing in both fields allowed him to make the connection.

"Sometimes it just takes someone to come along and join the dots," he said.

Licence said his findings change the understanding of Alexander's role in the events of 1066.

"This discovery therefore changes how we think about Alexander's motives in 1066," he said. 

"He wanted regime change in England."

The findings are detailed in Licence's new book, "Harold: Warrior King."