The Fellowship of the Ring is released on 19 December
The Parental Guidance classification means that while parents are advised some scenes may be unsuitable for children under eight, children of any age may see the film unaccompanied by an adult.
The Fellowship of the Ring is only the third film to carry a special consumer warning on its advertisements and publicity material.
It follows in the prehistoric footsteps of Jurassic Park and its sequel The Lost World: Jurassic Park - both of which were also rated PG.
The British Board of Film Classification, which awards the ratings, says children could find some of the battle and horror scenes disturbing.
The board says the film was not strong enough to be given the next classification up on the scale - a "12" rating - which would have limited cinema admission to children aged 12 and over.
But it describes the fantasy, adapted from writer JRR Tolkien's Lord of the Rings trilogy, as being "at the top end of the PG category".
The Fellowship of the Ring goes on general release on 19 December.
Some analysts predict that it will outsell the current fantasy blockbuster, Harry Potter and the Philosopher's Stone.
Potter fans suffered a similar period of uncertainty before that film, too, was made PG.
Though aimed primarily at children, the Potter film, about a boy wizard, contains dark episodes that might have led the BBFC to award a 12 rating.
The Fellowship of the Ring, which is almost three hours long, is awaited by more than 100 million Tolkien fans worldwide.
The Lord of the Rings tells the story of how the hobbit Frodo seeks to destroy a magic and much sought-after ring.
The novel is regularly listed in surveys of the most popular books in Britain.
The first film stars Sir Ian McKellan, Cate Blanchett, Ian Holm, Liv Tyler and Elijah Wood.
It is due to receive its world première in London on 10 December.