Some might say it is a court case worthy of its subject matter: impenetrable, verging on the farcical and wrapped up in the minutiae of Christian theology.

Amid the appropriately neo-gothic setting of the High Court in London, two British-based writers yesterday claimed that The Da Vinci Code, the loosely historical murder mystery, plagiarises a book they published more than 20 years earlier.

The two, who specialise in historical conjecture, claim that its author, Dan Brown, cannibalised their text, The Holy Blood and the Holy Grail, to give his book plausibility and to save himself "time and effort" in independent research.

Michael Baigent, 52, and Richard Leigh, 62, also said that it was not just random facts that were "lifted" but the whole "architecture" and "theme" of their book.

At the heart of the case is their theory that Christ did not die on the cross but married Mary Magdalene and had a child, starting a bloodline that was protected by the Knights Templar and hushed up by the Catholic Church.

Brown's thriller is also based on the notion that Jesus married Mary, starting a family in France where their descendants continue to live.

While the arguments in the case will hardly trouble historians, millions of pounds of publishing profits are at stake, as is the proposed release of the film version of The Da Vinci Code.

With sales of 40 million and counting since it was published in 2003, the book has become an international phenomenon, generating millions of pounds of publishing and tourism spin-offs.

The film, starring Tom Hanks, Sir Ian McKellen and Audrey Tautou, is due to be released in May.

Brown, a devout Christian who attended the case, emphatically denies stealing from Baigent and Leigh's work and is particularly adamant that he would never suggest that Jesus was not crucified on the cross.

In a statement Brown, 40, a reclusive figure from New Hampshire, said: "This is not an idea that I would have ever found appealing.

"Being raised a Christian and having sung in my church choir for 15 years, I am well aware of Christ's crucifixion and ultimate resurrection as the very core of the Christian faith."

He added: "The resurrection is perhaps the sole controversial Christian topic about which I would not desire to write.

"Suggesting that they marry Jesus is one thing but questioning the resurrection undermines the very heart of Christian belief."

Baigent, a New Zealander who moved to Britain 30 years ago, and Leigh, an American who also lives in this country, wrote their book in 1982 along with another author, Henry Lincoln, who has no part in the action.

Their book is a best-seller in its own right.

They claim that when The Da Vinci Code was first printed, many people in their field noticed the similarities between the books and they began legal action almost immediately.

The case, however, has taken three years to get to court.

In a hearing last year, Leigh said: "I don't begrudge Brown his success.

"I have no particular grievance against him except for the fact that he wrote a pretty bad novel."

Leigh and Baigent are suing Random House, the British publisher of The Da Vinci Code, which also published their work, for breach of copyright.

Brown's book, although roundly criticised - Salman Rushdie described it as a "book so bad it makes bad books look good" - has sold four million copies in this country.

They are suing for damages for past royalties and future earnings.

Jonathan Rayner James, QC, for the claimants, listed 15 points at which he claims that the "central theme" of the earlier book is copied in Brown's novel.

He also pointed out a number of pieces of texts that he claims are directly taken from one book to another.

He claims that Brown worked from notes researched by his wife Blythe to give "plausibility" to his work.

Mr Rayner James added: "It is not as though Brown has simply lifted a discrete series of raw facts from The Holy Blood and the Holy Grail.

"He has lifted the connections that join the points up. He and/or Blythe has intentionally used Holy Blood and the Holy Grail in order to save time and effort that independent research would have required."

The claimants also point out that one of the characters in the book, a museum curator, has the same surname as Berenger Sauniere, a real person who was extensively mentioned in The Holy Blood and the Holy Grail.

While Brown denies copyright infringement, he has already acknowledged a debt to the writers in the pages of his book.

One of the characters, Sir Leigh Teabing (an anagram of Baigent) picks the book off a shelf and gives his opinion of it.

"To my taste, the authors made some dubious leaps of faith in their analysis," he tells another character.

"But their fundamental premise is sound, and to their credit, they finally brought the idea of Christ's bloodline into the mainstream."

The case continues.