Freedom of speech online is under its fiercest threats in a decade because of two proposals in the US Congress, the Center for Democracy and Technology (CDT) said.

"Free speech online is facing some of its most serious assaults" since the Child Online Protection Act (COPA) was passed in late 1998, said Leslie Harris, executive director of CDT, a civil liberties advocacy group. One of those proposals would require schools and libraries to block internet chat and social networking tools.

The US government continues to spend millions of dollars to fight successful court challenges to COPA, which required adult-themed sites to get proof of age before allowing surfers to access adult content, the CDT said on Thursday.

On July 26, the House of Representatives passed the Deleting Online Predators Act (DOPA), which would ban social networking sites and instant messaging programs from US schools and libraries. A provision requiring sites with sexually related content to include warning labels is included in a bill awaiting action in the Senate.

Both proposals go too far in their attempts to protect children from online pornography or sexual predators, the CDT said.

The adult labelling provision requires any site with sexually explicit "depictions" to be labelled. The broad definition could mean that sites would have to include labels if they have text descriptions of sexual acts, sex education content or videos with no nudity, said John Morris, director of the CDT's Internet Standards, Technology and Policy Project.

The broad labelling requirement likely violates the free speech protections in the First Amendment of the US Constitution, Morris said. DOPA would add "a whole new category of social conversation" that is restricted speech, he said.

"99.999 per cent of instant messages that minors participate in are healthy," Morris said. "And they're perfectly legal."