The Da Vinci Code, the publishing phenomenon of the decade, this week passed sales of 4 million in the UK, adding to the 40 million copies sold around the world.

But while Michael Baigent and Richard Leigh's non-fiction work The Holy Blood and The Holy Grail — published in 1982 — may not have captured the popular imagination to anywhere near the same extent, it has experienced an "enormous jump" in sales in the past week, according to one industry observer.

Giles Elliott, of Bookseller magazine, said internet and bookshop sales had increased 745 per cent in the UK since the hearing began, from around 350 copies a week to more than 3,000.

"The trial means both books are getting worldwide attention and sales increases in the UK have been dramatic," he said.

The trial is being covered by journalists from the US, Europe and Japan, and Mr Elliott predicted the surge in sales will be repeated in markets around the world.

Mr Baigent and Mr Leigh are suing Random House, the publishers of both The Da Vinci Code and The Holy Blood and The Holy Grail, claiming the novel was an infringement of their copyright because it borrowed the "architecture" of their book.

Publishing houses and authors are notoriously reticent about their financial arrangements, but writers usually earn around ten per cent of the cover price of their books, Mr Elliott said.

But any boost in sales could be wiped out by the cost of losing the case. The hundreds of hours of painstaking comparison of the similarities and differences between the books by lawyers on both sides may push the final legal bill upwards of £2 million, some legal experts estimate.

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