PIRATES II: The most expensive porn film ever made will no longer be shown to students.

An attempt to distract US students from late-night drinking, with a feature-length porn movie, has been blocked after threats to cut college funding.

The two-and-a-half-hour Pirates II: Stagnetti's Revenge - the most expensive pornographic film ever made, at a cost of $US10 million ($NZ17.6 million) - would have been shown at a University of Maryland student union theatre on Saturday.

Organisers at the College Park campus, 16km north of Washington, had championed educational aspects of the screening, with health group Planned Parenthood planning to hold a presentation on safe sex practices beforehand.

The event was also previously seen by university officials as an "alternative to late-night drinking and other dangerous activities," the Baltimore Sun reported before the cancellation.

Republican state senator Andy Harris, however, proposed an amendment to the state budget to deny millions of dollars in funding for any educational institutions that screen a porn movie.

Harris said he had been "shocked and dismayed" to hear the college was going to screen the movie, and denounced what he described as the "dangers of pornography."

He said he was "extremely concerned that the policy of our public colleges and universities would allow `hard core' pornography" to be shown.

"I am pleased to know that the university did the right thing and cancelled this movie. However, I remain concerned that they do not have a policy prohibiting this," the senator said in a statement after the university reversed its decision.

Harris added he was "working to seek assurances that this will not happen again."

The Sun said that during a lengthy debate Thursday morning at the state legislature in Annapolis, Maryland, senate president Thomas Miller indicated he would back Harris' threat to cut millions of dollars in funding.

Linda Clement, the university's vice president of student affairs, denied the cancellation was linked to threats made by state lawmakers.

"No, we cancelled the (showing) because the educational context of the movie has been lost in the titillation that's been associated with the movie itself," according to Clement's spokesman Millree Williams.

"That's hard to believe," responded Adam Kissel, director of the Individual Rights Defence Program at the Foundation for Individual Rights in Education based in Philadelphia, Pennsylvania.

The university's claim, he said, was extremely unlikely because beforehand "university administrators had known about it, had expected it to go on and they had no problem with it."

Kissel said his education rights group was "very concerned" about the likely constitutional violation, namely the First Amendment that protects free speech.

"Strictly based on the plot and the trailer, the movie has plenty of action beyond the sexual action -- it has a plot, it has intrigue, just like any movie.

"So it's almost 100 percent certain that the university is violating the free speech rights of the students," said Kissel, noting that a First Amendment case was strengthened because of the educational component of Planned Parenthood's presentation.

Digital Playground, the adult film company behind "Pirates II," said on its website it had already shown the film to thousands of students on several college campuses this year, including the University of California, Los Angeles; Northwestern University in Chicago; and Southern Connecticut State University.

On-campus showings of X-rated movies such as "Behind the Green Door" and "Deep Throat" stirred sensation and controversy in the 1970s and 1980s, but porn has become commonplace on campuses over the past decade with virtually unlimited access to X-rated material via the internet.