The parents of Madeleine McCann are going to sue former Portuguese Police Chief Goncalo Amaral for defamation.
Kate and Gerry McCann are taking the action over comments made by the man who previously led the inquiry into Madeleine's disappearance.
In a statement, they said: "We - together with our three children Madeleine, Sean and Amelie - are taking this legal action against Goncalo Amaral over his entirely unfounded and grossly defamatory claims - made in all types of media, both within Portugal and beyond - that Madeleine is not only dead, but that we, her parents, were somehow involved in concealing her body."
The couple, from Rothley, Leicestershire, said they are taking action because his claims that Madeleine is dead are hampering the search for their daughter.
They want a gagging order to prevent further publication of Amaral's "deeply offensive" book "The Truth of the Lie."
The McCanns also want to block broadcast of his TV documentary and publication of his "disgraceful thesis" that they were involved in their daughter's disappearance.
The statement continued: "The primary reason for our legal action is simple: to stop any negative effect that these absurd and deeply hurtful claims may be having on the ongoing search for Madeleine."