Kathleen McGowan: The Da Vinci descendant

If you're one of the 40 million readers of The Da Vinci Code, get ready to stretch your credulity still further. An author is about to claim that she is the living embodiment of the Holy Grail, a direct descendant of the physical union between Mary Magdalene and Jesus Christ. She's American, she's 43, and she means it - every word.

Even though it seems her claim comes straight from the world of fictional make-believe, Kathleen McGowan, a married mother of three, is about to tell her story in a "partly autobiographical" novel that hopes to trade directly on the vast worldwide audience captured by Dan Brown. It promises to light almost as many fires of controversy as Brown's novel itself.

For, however outlandish Ms McGowan's claims, they are being taken seriously by one of the world's most respected publishing houses. Simon & Schuster is publishing her book and ploughing a marketing budget of more than a quarter of a million dollars into promoting it. It has already invested a seven-figure sum in the rights to her book, The Expected One. "I certainly expect there will be a backlash," said Ms McGowan. "But I have the support of my family and friends and that's what I draw from."

Claims to such a genetic lineage might normally draw scepticism from a world where an author's credibility can make or break a book. But rights to Ms McGowan's story of religious intrigue have been sold in more than 20 languages and the first print run in the US alone is 250,000. Her novel - which was first self-published last year and sold just 2,500 copies - goes on sale in Britain next month. It is the first part of a trilogy.

Digby Halsby, of Simon & Schuster's UK division, said Ms McGowan had been working on the book since 1989 and thus predated Dan Brown's efforts. When asked how a publisher could authenticate the author's claim about her heritage, he said: "It's impossible to verify. It's all to do with a matter of faith. She makes a very convincing argument."

Academics are highly sceptical of claims that Mary Magdalene travelled to France and that she and Jesus had a child. Bart Ehrman, chair of the department of religious studies at the University of North Carolina and author of Peter, Paul and Mary Magdalene: The Followers of Jesus in History and Legend, said: "You can survey anyone who is a scholar of early Christianity and they will tell you the same thing - it's completely bogus. There are no records. We have no account of Mary Magdalene even going to France until the Middle Ages and the legend about her going to France sprung up because there was a cult to Mary Magdalene in southern France."

Ms McGowan's claim to be descended from a child of Mary and Jesus is outlined in the afterword to what she says is a partly autobiographical novel.

She has offered no proof of her heritage but said she had traced it to an ancient French lineage that claimed to trace its roots to the pair.

Two books, one story

'The Da Vinci Code' claims that for centuries the secretive Catholic group Opus Dei has been doing all it can to continue to keep secret the fact that Christ had a child - and that the bloodline continues until this day. Kathleen McGowan claims to be part of this bloodline.

In Christian mythology, the Holy Grail was the chalice or cup that was used by Jesus at the Last Supper and is said to possess miraculous powers. Others say it was the vessel used to collect Christ's blood at the crucifixion. But according to The Da Vinci Code, the description of a chalice is merely an allegory to protect the real Grail.

Dan Brown's book and the earlier Holy Blood, Holy Grail - which was written 20 years previously, and whose authors mounted an unsuccessful legal action against Brown for plagiarism earlier this year- suggest that Christ married Mary Magdalene and together they bore children. The writers of both books say it is Magdalene herself who is the Holy Grail - the "sacred feminine", the vessel that carried Jesus's children.

Brown's book maintains that after the crucifixion Mary, who was pregnant, moved to France with the help of Jesus's uncle, Joseph of Arimathea. She found refuge in the Jewish community and gave birth to their daughter, Sarah.