Hundreds of full-length feature films including blockbusters from Walt Disney Co. DIS +0.55% and Sony Corp.'s 6758.TO -10.14% Columbia and Tristar studios have been illegally uploaded to the world's most popular video site, generating hundreds of millions of views over the past year.

Why the movie studios didn't block the films by using a special YouTube program—called Content ID—for identifying their copyrighted content is a mystery.

On Thursday, after inquiries by The Wall Street Journal, Disney used Content ID to successfully block some of the classic animated films that have been on YouTube for as long as a year, including "Peter Pan," "Snow White and the Seven Dwarfs," and "Fantasia."

Representatives of Disney, Sony, MGM Studios and Warner Bros. declined to comment on the situation or why they haven't blocked the films using Content ID.

A YouTube spokeswoman said the site "invested heavily in copyright and content management tools to give rights holders control of their content on YouTube" and that more than 4,000 media companies use Content ID. The site is continually improving the technology behind Content ID, she said, adding that to date it has identified more than 200 million copyrighted videos that were uploaded.

Howard Gantman, a spokesman for the Motion Picture Association of America, a trade group that represents film studios in copyright matters, said: "We are aware of the issue and are concerned about it. Our member companies have raised the issue with YouTube and hope they will work cooperatively with us to fix it."

The return of the problem shows how piracy continues to pop up for YouTube, even after it took steps to eradicate the issue and the problem had lain largely dormant over the past five years.

YouTube, acquired by Google in 2006, was sued in 2007 by Viacom Inc., VIAB +0.66% which owns TV channels such as MTV. Viacom claimed there were thousands of unauthorized clips uploaded to YouTube and Google was liable for copyright infringement.

YouTube said it is protected from liability by a law called the Digital Millennium Copyright Act, and that it took down infringing content when it received notices. The suit is still pending.

Google in late 2007 launched Content ID, which helps identify copyrighted material so content owners can block the content from appearing on the site altogether, or in certain geographical markets. Alternatively, content owners can choose to let YouTube sell online advertisements that appear next to or on top of the videos while they play. Content owners get the majority of the ad revenue.

Some of the full-length films uploaded by individual users within the past year, including some produced by MGM Studios such as the 1990 hit "Misery," currently appear with ads.

Content ID helped curtail the piracy issue, some media executives have said. Since then, Google has struck deals with Disney and other studios to host some of their TV content on YouTube or sell movie rentals of full-length feature films.

The recent problematic uploads may have undercut the rental effort. Movies produced by Disney's Touchstone Pictures, such as "I Am Number Four" and "Shanghai Noon" are available free on YouTube, even though the studio struck a rental deal through the site.

To be sure, illegally uploaded movies could easily become unavailable if the content owners attack the problem.

Some individuals who uploaded films using pseudonymous accounts wrote in the video description box on the Web page that they don't own a copyright. Others provide written instructions to other YouTube users on how to download the illegally uploaded films from YouTube using certain software.

People who have used Content ID at media companies said the system works well, but that some individuals who illegally upload videos to YouTube have been able to manipulate the soundtrack or video to avoid detection by the system. These people add that some media companies don't always use the system properly and forget to block some illegal content.

On social media sites such as Twitter, many people have expressed surprise about the full-length films and are posting links to their followers.

Kailey Fry, a high-school student near Toledo, Ohio, said she previously saw written notices on YouTube pages that pirated content had been removed due to copyright claims. But several months ago, a friend told her that YouTube had a growing array of full-length films.

Last week, Ms. Fry discovered and watched the 1957 film, "The Three Faces of Eve" on YouTube. Since then she has watched other films and organized a sleepover for friends to watch "Halloween," the 1970s horror flick, on YouTube using her laptop. "I'm going to use YouTube a lot more," the 16-year-old said.