The illegal downloading of movies on the Internet is up 20% over last year, according to a new study released by Internet research firm Viant.

With figures that should give Motion Picture Assn. of America prexy Jack Valenti much fodder in his anti-piracy crusade, Viant estimates that between 400,000 and 600,000 films are downloaded daily on the Internet from sites such as Kazaa, Grokster and Morpheus. Last year, Viant said between 300,000 and 500,000 pics were illegally downloaded.

The number of file-swappers online at one time soared to more than 9 million in May, with most downloads focusing on “Spider-Man” and “Star Wars: Episode II — Attack of the Clones.”

A “cam” version of “Spider-Man” (captured by a video camera in the audience of a pre-release screening) was first detected the evening of May 2 and quickly spread on May 3, the day the film opened in theaters. “Star Wars,” also in cam form, first appeared on May 8, and trading became widespread on May 9, six days before the film’s opening in theaters, the report said.

Technology constraints continue to hamper most downloads, however. Viant estimated that of the nearly 10 million people who sought bootleg copies of “Star Wars” and “Spider-Man” on the Internet, only about 3 million were successful in obtaining complete copies of either.