Fascination with the court case has fuelled a massive surge in sales for Holy Blood, Holy Grail, the book at the centre of the case which is said to be the basis for Brown's bestseller. Its writers, Michael Baigent and Richard Leigh, have witnessed up to a 10-fold rise in sales in the days since the court case began last week.

One luxury version of the book . . . an illustrated edition costing 30 . . . saw its sales rocket by 3,500% on Amazon in the 24 hours after the case began. It is not just online that the increase is apparent. By midweek, Waterstones had seen a boost of 620% for Holy Blood on the previous week's figure.

On Monday the authors on both sides strode through the media circus into the High Court, buffered by a ring of publicists and minders, as their legal teams fired their opening salvos. Even as their lawyers were making their initial submissions, customers were tapping out their credit card details or trooping into shops to stock up on the book.

After years of modest sales, Brown's blockbuster has already turned Holy Blood into a money-spinner with fans reading up on the book's central conspiracy theory. The paperback was selling an average of around 3,500 copies a year prior to publication of The Da Vinci Code. But that figure quadrupled in 2004 in the wake of Brown's success, then doubled again to 31,341 sales in 2005.

In the past seven years, the book has sold 83,000 copies and generated three-quarters of a million pounds in the UK.

In the run up to the case, Holy Blood was still shifting in the region of 500 copies a week.

On the first day of last week's court case, sales eclipsed that, selling up to 1,000 copies in the 24 hours.

Cathy Waterhouse, senior bookseller for London's Oxford Street branch of Waterstones said: "The court case seems to have sparked a huge amount of interest in both books. One of the most amazing things abut The Da Vinci Code is that surely you would think everyone must own it by now, but it still keeps flying off the shelves. I don't know how we keep selling in such huge volumes. Everyone must have bought one, but still it keeps on selling. I spoke to one customer who had already read it, but bought another copy just so he could read it again."

Leigh and Baigent brought their action for breach of copyright against publisher Random House, which also publishes their own book, claiming Brown stole the ideas for his story from their investigation, which was published in 1982.

They say that Brown's pageturner took the "architecture" of their book . . . that Jesus did not die on the cross but married Mary Magdalene and started a bloodline . . . on the back of their hard work.

But the publisher's QC John Baldwin rebutted their claim in court, saying there could be no copyright in an idea, and that some of their theories had originated before the publication of their book. The judge adjourned the case on Wednesday to allow him to read the works.

An editorial in this week's edition of the publishing journal The Bookseller declared that if it were shown to be a copy, it would have a profound effect on Brown's reputation.

"The charge of plagiarism is a deadly cancer to a writer's reputation, and if the court so brands Brown, the damage will be just as great as if he were a literary author or historian. Make no mistake: Dan Brown, plagiarist, has far less appeal to the book-buying public. Random House would have little choice but to urgently find grounds for an appeal, " it said.

If the writers win, they are expected to seek a share of Brown's royalties and could injunct the release of a film version of The Da Vinci Code starring Tom Hanks.

The case has also given a fillip to Brown's sales, which have risen fourfold. His UK sales will pass four million this week.