Carriage Services Inc., a Houston funeral-services company, recently discovered that 70 percent of the workers in its 125-person headquarters watched videos on Web sites like Google Inc.'s YouTube and News Corp.'s MySpace for about an hour a day.

"I almost fell out of my chair when I saw how many people were doing it and how much bandwidth those sites sucked up," says Jeff Parker, the company's information-technology administrator. He quickly blocked access to both sites.

Like Carriage Services, companies across the U.S. are starting to prevent their employees from accessing Internet-video services at work. The move follows previous steps by IT departments to shut employees' access to instant-messaging services, streaming music and Web sites with adult content.

Now, online video has become an increasing irritation. Worker productivity is being jeopardized as short, often low-quality video clips popularized by YouTube are being joined by better-quality video services with long-form content. According to a study released last month by Nielsen Online, an Internet tracking service owned by Nielsen Co., the heaviest consumption of Internet video is during weekday lunch hours between 12 p.m. and 2 p.m., when most people are at work.

Online video also is taxing already-strained corporate-technology networks. It poses a particular problem for smaller companies, which have limited bandwidth capacity to accommodate bulky video files. Online video files on average are about seven times as large as audio files and 100 times as large as e-mail.

In December, Internet users watched more than 10 billion videos online, according to comScore Inc. — one of the single heaviest months for online-video consumption since comScore began tracking it in 2006. And with Web sites such as Hulu LLC and Netflix Inc. set to roll out heftier high-definition video services in the coming months, corporate networks face slowdown in computer traffic and possible outages.

For companies that have a limited amount of bandwidth, Internet video can be a significant drain on resources, says Paul Stamp, an analyst with Forrester Research. "Without having some kind of a set policy that either controls or blocks video, (companies) run the risk of their networks crashing or, at the least, slowing down drastically."