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.

Scrabulous was created in July 2006 by Rajat and Jayant Agarwalla from Kolkata, India, as an online version of the game, using a colour-coded board similar to Scrabble's.

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.

Hasbro owns the U.S. and Canadian rights to Scrabble, while Mattel owns the rights to the game in the rest of the world.

"Letters have been sent to Facebook in the U.S. regarding the Scrabulous application," Mattel said in a statement.

"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."

A spokesperson for Hasbro Canada confirmed the joint statement, calling Scrabulous "a gross copyright and trademark infringement of Hasbro’s SCRABBLE rights in the U.S. and Canada."

"We are not surprised that fans have thoroughly enjoyed playing Scrabulous on Facebook.com," said Hasbro Canada .

"What consumers may not realize, however, is that Scrabulous is an illegally copied online version of the world’s most popular word game. We encourage fans to continue to lay down online tiles at sites that have legally licensed the interactive rights to host SCRABBLE fun," the company said in a statement.

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.

Other sites blamed for copyright infringements

The news is not surprising to John Chew, the director for the Canadian National Scrabble Championship. Chew said Scrabulous is not the first internet server to host the Scrabble game without permission, and the sites are usually shut down after receiving a cease-and-desist letter from Hasbro or Mattel.

What makes the Scrabulous situation unique is its popularity on Facebook.

"Scrabulous has become a victim of its own success," said Chew, who also runs the Toronto Scrabble Club.

Through his work with the Canadian championship, Chew has contractual ties to Hasbro, but, like many Scrabble players, he also uses the Scrabulous application.

He said he hopes the issue can be resolved.

"As a fan I hope they can work out some sort of licensing deal," he said. "Online versions of the game have been good for the game of Scrabble and have helped contribute to the mainstream appeal of the board game."