The American Civil Liberties Union (ACLU) has asked school board officials in Tennessee to stop censoring gay websites.

Schools in the area use a web filtering system that censors out unsuitable material such as websites which have sexual content.

However, the filter also blocks gay educational websites which contain no inappropriate content, such as the Gay and Lesbian Alliance Against Defamation and Marriage Equality USA.

In a letter sent to schools by the ACLU, it says the ban on gay websites is illegal. It has said it will file a lawsuit if the school districts do not come up with a plan to restore access to the LGBT sites by the beginning of the 2009-2010 school year.

The organisation is not challenging Tennessee law, which requires schools to filter out material that is obscene or harmful to minors

“Students at Knox County and Metro Nashville schools are being denied access to content that is protected speech under the First Amendment as well as the Tennessee state constitution,” said Tricia Herzfeld, staff attorney with the ACLU of Tennessee.

“This kind of censorship does nothing but hurt students, whether they’re being harassed at school and want to know about their legal rights or are just trying to finish an assignment for a class.”

Andrew Emitt, a 17-year-old senior at Central High School in Knoxville, said: “When I found out about this web filtering software, I wasn’t looking for anything sexual or inappropriate – I was looking for information about scholarships for LGBT students, and I couldn’t get to it because of this software.

“Our schools shouldn’t be keeping students in the dark about LGBT organisations and resources.”

“One of the problems with this software is that it only allows students access to one side of information about topics that are part of the public debate right now, like marriage for same-sex couples,” said Karyn Storts-Brinks, a librarian at Fulton High School in Knoxville.

She added that the software blocks access to organisations that support marriage for same-sex couples like the Religious Coalition for Freedom to Marry or the Interfaith Working Group while allowing access to websites that oppose marriage equality.

Although gay charities and similar organisations are blocked, websites that urge gay or trans people to change their sexual orientation or gender identity through so-called “reparative therapy” or “ex-gay” ministries are still permitted.