Dan Brown, whose The Da Vinci Code has sold an estimated 40 million copies, went into the witness box at the High Court in London yesterday to defend his publishers against accusations that he lifted his ideas from a book published more than 20 years previously.

Dressed in a dark suit, blue shirt and yellow tie, Brown does not look like one of America’s richest authors. He would pass more easily for a computer geek or a mathematics teacher, the profession of his father.

He speaks with a cultivated New England accent, is polite and patient, and displayed irritation only when Jonathan Rayner-James, QC, appearing for the authors of The Holy Blood and The Holy Grail, asked him questions so convoluted that he could not understand them. Everyone else in the crowded courtroom had similar trouble, except Mr Justice Peter Smith, who tried to explain to Brown the purpose of the more abstruse questions.

“What exactly are you asking me?” Brown said several times when the questioning became manifestly less clear than the galloping narrative of his novel. What Mr Rayner-James seems to be asking is at what stage Brown consulted HBHG, a work of historical conjecture, which broadly explores some of the heretical theories of early Christianity that are touched on in DVC. Michael Baigent and Richard Leigh, the co-authors of HBHG, are suing for infringement of copyright.

The line of questioning became tediously detailed on the matter of when Brown read the 300 research documents and 40 books, including HBHG, which he admits to consulting for DVC. But in cross-examination and his 69-page witness statement, he dropped a few handy hints on how to become a successful author of code-based thrillers.

You have to have a mathematically minded father, who, instead of leaving your childhood Christmas presents under the tree, makes you solve puzzles and anagrams to find them. You need a wife sufficiently enthused to undertake mountains of research, annotate her findings and put them on your computer. Blythe Brown, who is not in court, is that woman.

Then you have to find the right location. Brown once considered setting a thriller in Canada but found Nova Scotia lacking in mystery. He had been far more enthused, during a holiday in Rome, by a tour of the Vatican and by finding a secret exit through which a pope could escape from his enemies. The secrets of the Eternal City inspired his next novel Angels and and Demons. “Location is a character. After that trip I decided the Vatican would make a great character, ” Brown said.

After that, you have to have the Big Idea. In DVC, Brown said, that was the Sacred Feminine, or goddess worship, a notion he claims he first came across in a book called The Woman with the Alabaster Jar, and not HBHG. Having had the Big Idea, Brown began to construct a framework for a plot. “The ideas are the easy part; ideas are everywhere. The hard part is getting the ideas to work as a novel,” he said. He sent a synopsis to his agent and suggested two more thrillers on the same theme, provisionally called The Botticelli Code and The Nostradamus Code.

Unwilling to give a precise order in which he consulted his sources, Brown explained that an author was likely to refer backwards and forwards between sources. Sometimes material emerged which set the plot in a new direction. Brown said that one of the themes of DVC was “secret history”, those parts of the past lost or twisted by historical revision or subversion. “I like to write in grey areas; I don’t like the idea of black and white, right and wrong.”

At first he had been unwilling to introduce the idea of Jesus’s bloodline into DVC because he found it “too incredible and too inaccessible”. But his wife persuaded him to adopt the theme, which he had read about in many sources before HBHG. His statement added: “I remain astounded by the claimants’ choice to file this plagiarism suit. For them to suggest . . . that I have ‘hijacked and exploited’ their work is simply untrue.”

The hearing continues.

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