Rupert Murdoch's Fox News channel is being taken to the federal trade commission over claims that its boast of being "fair and balanced" is a fraud.

Two pressure groups - the liberal internet-based group MoveOn.org and the historical non-partisan Common Cause say its news reports were "deliberately and consistently distorted and twisted to promote the Republican party of the US and an extreme rightwing viewpoint".

Fox News's Irena Briganti told the Associated Press news agency that the move was "clearly a transparent publicity stunt" by the pressure groups.

In a statement, the commission's chairman, Timothy Muris, indicated that the petition stood little chance of succeeding.

This is not the first time that Fox's motto has met with controversy. When the liberal comedian Al Franken brought out his book Lies and the Lying Liars who Tell Them: A Fair and Balanced Look at the Right, Fox News tried to block publication, claiming the book made unauthorised use of the slogan.

The judge in that case dismissed Fox's case, saying it was "wholly without merit", and that the channel's trademark Fair and Balanced, registered in 1998, was weak.

Since then Fox's request to trademark the phrase has been challenged by the Independent Media Institute, which claims the term is so prevalent as to be generic, and is "entirely misdescriptive" when it comes to the channel.