London's High Court in April ruled in favor of Brown and publisher Random House and ordered historians Michael Baigent and Richard Leigh to cover the victors' court fees of nearly $2 million in addition to their own $1.5 million in court costs.
During testimony, Brown called the plagiarism charge "absurd."
Both books are based on the theory that Jesus had a child with Mary Magdalene whose descendants still survive, but Baigent and Leigh's book was non-fiction while Brown's novel was a worldwide bestselling thriller. Both were published by Random House.
Ironically, the popularity of Brown's novel has goosed sales of the "Holy Blood, Holy Grail," Variety noted.
No date has been set for the appeal, which is expected to start in 2007.