Kate and Gerry McCann announced this weekend they are to sue Goncalo Amaral for defamation after he claimed in a book and series of interviews that they were involved in the little girl's death.

The couple, from Rothley, Leicestershire, said they are taking action because his claims that Madeleine is dead are hampering the search for their daughter.

His comments had also caused "indescribable devastation and suffering" to their lives, they said.

But Mr Amaral, who was in charge when the McCanns were made official suspects and was later removed from the case, said he plans to counter-sue the pair.

"My lawyers are working on the case and we will take the McCann couple to court and perhaps other people for defamation, slanderous denunciations and false statements," he said on Portuguese TV.

The McCanns want a gagging order to prevent further publication of Mr Amaral's "deeply offensive" book The Truth of the Lie.

They also aim to block broadcast of his TV documentary and publication of his "disgraceful thesis" that they were involved in their daughter's disappearance.

They said in a statement: "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.

"We can no longer stand back and watch as Mr Amaral tries to convince the entire world that Madeleine is dead.

"Nor can we allow this blatant injustice to Madeleine, with its obvious risk of hindering our attempts to find her, to continue."

Madeleine disappeared two years ago in Praia da Luz, Portugal, while her parents dined in a nearby tapas bar.

The couple recently marked the second anniversary with a PR offensive incorporating renewed appeals for information into Madeleine's disappearance.

They will join families of other vanished youngsters at London's South Bank on Monday to mark International Missing Children's Day.