One of the most high-profile and potentially far-reaching copyright cases seen in the English courts recently will start this morning, with two authors of a non-fiction book suing Random House, publisher of Dan Brown's best-seller The Da Vinci Code, over alleged infringement.

Michael Baigent and Richard Leigh are claiming that the American author's novel appropriated themes and ideas that they explored in a 1980s work called The Holy Blood and the Holy Grail.

These included the contention that Christ might have married Mary Magdalene and, after he survived the crucifixion, they travelled to France with their children. As a result, they could have had a direct bloodline, protected by the Knights Templar and other groups carrying on their work.

Another of the book's suggestions, similarly inflammatory, was that the Roman Catholic church might have tried to kill remnants of this dynasty during the Inquisition to maintain power through the apostolic, rather than hereditary, succession.

Two decades ago clergymen labelled the work "obnoxious" and "academically absurd", but the notoriety prompted brisk sales.

Dan Brown's book, on the other hand, is sold as fiction and has sold tens of millions of copies. The second of a trilogy, it deals with the efforts of a Harvard professor to solve the murder of a curator at the Louvre museum in Paris. This, in turn, leads on to claims that the Catholic Church has been involved in a plot to conceal the true story about Jesus and his bloodline.

Included in the novel is a character called Sir Leigh Teabing, a historian, whose surname is an anagram of "Baigent".

Lawyers say that legal cases involving claims of stolen ideas rather than more direct forms of copying are always likely to face an uphill task in the courts, but they believe the forthcoming battle could have widespread implications for the use of non-fiction research in other books.

"Cases involving copyright infringement are notoriously difficult to prove," says Antony Gold, head of contentious intellectual property at Eversheds.

"The claimant needs to show clear examples of similarities and in most cases has to narrow these down to specific incidents where an idea has been directly copied."

But he adds: "What makes this case interesting is that there is little clarity over the extent to which an author can use another person's research for either background or as a direct influence on a book."

Guy Tritton, an intellectual property specialist and barrister at Hogarth Chambers, agrees that The Da Vinci Code tussle will be "a very interesting case which falls in borderline territory of what is protected".

The case, he suggests, pits the rights of authors to write new books but utilise earlier research, against protections for non-fiction writers who do not want to see their labour and effort misappropriated.

Dan Brown has already triumphed in another legal battle in the New York courts, where a judge found that there was no "substantial similarity" between his book and two novels by another writer, Lewis Perdue.

But Mr Tritton points to an earlier English case, which has some parallels to the current situation, which did end in victory for the claimant alleging infringement.

In the 1980s, author James Herbert was successfully sued over his novel The Spear and its links to Trevor Ravenscroft's The Spear of Destiny.

In the current case, the High Court in London is expected to begin hearing opening arguments this morning.

However, Mr Justice Peter Smith is expected to take time to read the material involved before witnesses, including Dan Brown, give evidence.

If Random House were to lose the case, lawyers said it was likely some form of royalty sharing deal would have to be negotiated or imposed.