Another day, another conspiracy theory. Dan Brown’s bestseller The Da Vinci Code outraged traditionalists with the idea that Jesus married and had children. The theological applecart is about to be upset again by a new scholarly book that kicks down another central prop of Christianity. It claims Jesus was intent on founding a royal dynasty, not a new religion.

With The Jesus Dynasty, published next month, the biblical archeologist James Tabor is tapping into a market that has made a multi-millionaire out of Brown, who was being sued in the High Court last week by Michael Baigent and Richard Leigh. They claim that much of Brown’s novel derives from their 1982 bestseller The Holy Blood and the Holy Grail.

Brown’s book revolves around the theory that Jesus and Mary Magdalene married and founded the Merovingian dynasty of French kings, a secret protected by the Knights Templar and a mysterious group called the Priory of Sion, which included in its membership Leonardo da Vinci. Da Vinci supposedly left a “clue” in his painting The Last Supper by depicting the apostle John as Magdalene.

Tabor, chairman of the department of religious studies at the University of North Carolina, says such speculation is “highly suspect at best”, although he does not rule out the possibility that Jesus married and had children.

From archeological and textual evidence, Tabor has concluded that far from setting himself up as the Messiah, Jesus was intent on establishing himself and his family as the rightful rulers of Israel.

Jesus and his extended family were “royals” descended from King David, who ruled in the 10th century BC, Tabor says. He was proclaimed “king of the Jews” and executed for this claim.

Yet his paternity remains open to debate. His adoptive father Joseph apparently died childless and his mother Mary remarried Joseph’s brother Clopas, but there is “good reason to doubt” whether either of Mary’s two husbands fathered Jesus, Tabor maintains.

So who was his father? According to an anti-Christian work by the philosopher Celsus in AD178, Mary “was pregnant by a Roman soldier named Panthera and was driven away by her husband as an adulterer”. Such was the gossip in Jewish circles. In Germany Tabor tracked down the grave of a Roman soldier of the same name, possibly Jewish, who was a contemporary of Mary.

“So we have the right name, the right occupation, the right place and the right time,” Tabor concludes. “(But) there is no way to prove a connection with this type of evidence, short of DNA tests of identifiable remains.”

Contrary to assumptions that he came to found a new religion that would supersede Judaism, Jesus preached “a very Jewish apocalyptic message”. He wanted a social revolution, informed by spiritual values, in anticipation of the imminent collapse of the Roman empire, Tabor writes.

In the event, after Jesus’s crucifixion in AD30 his half-brother James became head of the family until he was executed in AD62 and was followed by Jesus’s second brother Simon, who ruled for 45 years before his brutal death. Leadership then passed to a man named Judas, perhaps one of Jesus’s nephews.

“What we clearly have is nothing less than a Jesus dynasty, taking us well into the 2nd century AD,” Tabor claims.

However diligent his research, Tabor’s theory is largely speculative, adding to the canon of colourful books that occupy a lucrative publishing niche. In this netherworld where mysteries intersect, nothing is off limits. Certainly not Diana, Princess of Wales.