Madeleine McCann's parents will lodge a complaint with Portuguese police alleging that former detective Goncalo Amaral broke his country's strict judicial secrecy laws, the couple's lawyer said.
Kate and Gerry McCann are already embroiled in legal action against Mr Amaral in an attempt to ban his book which claims their daughter is dead.
Now the couple intend to accuse him formally of passing on information about the police investigation into Madeleine's disappearance before the case was closed - a criminal offence in Portugal.
The McCanns' Portuguese lawyer, Isabel Duarte, alleged that Mr Amaral broke the law by sending a draft of his book to his publishers several months before the judicial secrecy period in the case was lifted in July 2008.
The lawyer also alleged that the ex-policeman released facts about the investigation that were not in the case files.
It will be for the Portuguese public prosecutor to decide whether the former detective has a case to answer.
Mr and Mrs McCann, both 41, from Rothley, Leicestershire, flew to Portugal this week to hear Mr Amaral attempt to overturn their ban on his book, Maddie: The Truth Of The Lie.
The judge, Maria Gabriela Cunha Rodrigues, is expected to send her ruling directly to the McCanns and Mr Amaral before the end of February. There will then be a full trial on whether the injunction banning the book should be made permanent at a later date, Ms Duarte said.
The McCanns are also seeking 1.2 million euro (£1.08 million) in compensation for defamation in separate proceedings against Mr Amaral in Portugal.
