Lawyers representing Kate and Gerry McCann have demanded a website claiming their daughter was not abducted be taken down.
The Madeleine Foundation claims Madeleine is dead and her parents bear some responsibility for her death.
In a statement on its website, the foundation says libel specialists Carter-Ruck have also demanded they hand over all remaining copies of leaflets and booklets they published to support their claims.
The spokesman for the McCann family, Clarence Mitchell, said: "Kate and Gerry have consistently made it clear they will not dignify the actions of the so-called Madeleine Foundation with any comment whatsoever.
"However Carter-Ruck's position is now clear and it is in the hands of Kate and Gerry's defamation lawyers."
The statement on the foundation's website states it was given until yesterday to reply to Carter-Ruck's letters and was now taking legal advice.