WASHINGTON — Tucked away in a spending bill pending in Congress is language the porn industry finds offensive.

Along with billions of dollars in funding for the Justice Department and the State Department is a little-noticed provision that would require Web sites that feature “sexually explicit” content to contain special labels to make it easier to keep kids away from them.

The provision’s backers — led by Attorney General Alberto Gonzales — say the language is a simple and logical step to keep kids a safe distance from porn and give parents greater control over their children’s online travels.

The adult entertainment industry, joined by civil libertarians, says the labels would be a digital-age scarlet letter that would violate constitutional free speech protections. Opponents argue that labeling should be voluntary and, besides, a major portion of the Internet’s X-rated sites are based overseas and are thus beyond the reach of U.S. law.

“It would impose criminal penalties on operators of sites for the lack of labeling on content that is constitutionally protected,” said Leslie Harris, executive director of the Center for Democracy and Technology. “If you consider the vastness of the Internet, it potentially makes criminals out of an extraordinarily diverse set of content providers.”

One of the bill’s weaknesses, opponents say, is that it does not clearly articulate what kind of content would be defined as “sexually explicit.” According to Harris, sites featuring the Victoria’s Secret lingerie catalog or exhibitions of erotic art could find themselves exposed to criminal penalties.

Harris also noted the language was never the subject of committee hearings in the Senate or the House, was never approved by any committee and was simply inserted into the bill as it headed to the Senate floor.

Under the provisions, all Web pages that contain “sexually explicit” content would require a special label that would make them instantly recognizable to filtering software. In addition, the language would ban adult content from the opening page of adult sites so anyone inadvertently arriving there would have the chance to leave before being exposed to any flesh.

“When a child, for example, accidentally types in a `.com’ instead of `.gov’ or `.org,’ sexually explicit photos would appear on the screen. This amendment will prevent that from happening,” Sen. Conrad Burn (R-Mont.), who sponsored the language, said last month when the language was shoe-horned into the spending package.

It was a long-running irony in Washington that seeking information on the presidency from www.whitehouse.com, instead of www.whitehouse.gov, would land the seeker at a porn site. That has since changed.

Though Congress adjourned earlier this month without passing the bill, both its supporters and its opponents have little doubt it will be back in January, when lawmakers reconvene after the holidays.

According to the adult entertainment industry group the Free Speech Coalition, most adult Web sites already require an affirmative act, like an additional mouse click, to access X-rated content. In addition, the industry is working on a set of voluntary “best practices” that include labeling all pages.

The Association of Sites Advocating Child Protection — a group funded by Playboy.com, Hustler.com, Livesex.com and dozens of similar adult-oriented sites — is promoting its RTA, Restricted to Adults, labeling system to “better enable parental filtering, and to demonstrate the online adult industry’s commitment to helping parents prevent children from viewing age-inappropriate content.”

The labeling provision inserted in the congressional spending bill grew out of a hard-hitting, at times graphic, speech given by Gonzales at the National Center for Missing and Exploited Children in Alexandria, Va., last spring during which he called for legislation to attack child pornography and keep kids from accidentally encountering adult content online.

He said the bill envisioned by the administration would “prevent people from inadvertently stumbling across pornographic images on the Internet. I hope Congress will take up this legislation promptly.”

The challenge was accepted by Burns, who was defeated in last month’s elections, and Sen. John Kerry, D-Mass. The two failed in their initial bid to get the language included in a stalled telecommunications bill, but they succeeded this fall in getting it included in the spending bill.

“We saw this as the best opportunity to ensure these needed protections are enacted,” Burns said.

The provision also contains noncontroversial language sought by the administration that would impose fines of $50,000 to $300,000 for Internet Service Providers or others who do not report users who are downloading or accessing child pornography.

“This amendment increases penalties for folks who turn a blind eye to any attempt to exploit children over the Internet,” Burns said.

Opponents of the labeling language scored a temporary victory when Congress punted the spending bill until January, leaving it up to the next Congress — which will be controlled by the Democrats — to reconsider the provision. Still, the language has the support of a number of influential Democrats.

“We fully expect it to be reintroduced, but we feel much more confident about next year,” said Harris of the Center for Democracy and Technology.