The makers of Scrabble have asked Facebook to remove a popular add-on to the social networking site that allows users to play an online version of the board game.

Lawyers for toy makers Hasbro and Mattel said the Scrabulous program infringes on their copyright for the word-based board game.

The program's popularity really took off when it was added as an application to Facebook in 2007. It now regularly attracts nearly 600,000 daily users and is listed as one of Facebook's 10 most popular applications.

"Mattel values its intellectual property and actively protects its brands and trademarks. As Mattel owns the rights to the Scrabble trademark outside the U.S. and Canada, we are currently reviewing our position regarding other countries."

In response to the news of the cease-and-desist letter, fans of the online game started a "Save Scrabulous" group on Facebook, which has attracted more than 5,000 members, most of whom joined after the BBC ran a story on the dispute.