Kate and Gerry McCann have signed a deal to write a book about the disappearance of their daughter Madeleine, their publishers announced today.
After a major bidding war, Transworld won the rights to the couple's account of how the little girl vanished on a family holiday to Portugal in 2007 and their efforts to find her.
All proceeds from the book will go to the McCanns' official fund to look for Madeleine, which had been in danger of running out next year.
Precise details of the publishing deal have not been released, but a source said it included a "substantial" advance and "enhanced royalties" giving the couple a bigger share of profits from sales.
The book, simply entitled Madeleine, will be published on April 28 next year to coincide with the fourth anniversary of the child's disappearance.
Mrs McCann, 42, said: "My reason for writing is simple - to give an account of the truth.
"Publishing this book has been a very difficult decision and is one that we have taken after much deliberation and with a very heavy heart.
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"However, in the last few months, with the depletion of Madeleine's Fund, it is a decision that has virtually been taken out of our hands.
"Every penny we raise through its sales will be spent on our search for Madeleine. Nothing is more important to us than finding our little girl."
Her husband, also 42, added: "We are hopeful that this book may help the investigation to find Madeleine in other ways too.
"Our hope is that it may prompt those who have relevant information - knowingly or not - to come forward and share it with our team. Somebody holds that key piece of the jigsaw."
Transworld has acquired the rights to publish the book - which is being written by the McCanns themselves and is currently part-complete - in the UK, the Commonwealth and Europe.
The west London-based publisher is home to best-selling authors including The Da Vinci Code writer Dan Brown, fantasy novelist Terry Pratchett and theoretical physicist Stephen Hawking.
A source said the McCanns' book, which will have a recommended retail price of £20, was expected to be a big seller.
Transworld publisher Bill Scott-Kerr said: "It is an enormous privilege to be publishing this book.
"We are so pleased to be joining Kate and Gerry McCann in the Find Madeleine campaign."
Deals for newspaper serialisations and translations of the book are now expected to be negotiated.
The McCanns, from Rothley, Leicestershire, are fighting a legal battle against former Portuguese detective Goncalo Amaral over his book alleging that Madeleine died in the apartment and that her parents faked her abduction - something they strongly deny.
The appeal court in Lisbon last month overturned an injunction obtained by the couple banning publication of Mr Amaral's work, Maddie: The Truth Of The Lie.
But a source close to the McCanns said their decision to write their own book had nothing to do with rebutting Mr Amaral's allegations.
"It certainly hasn't been prompted by him," the source said.
Madeleine was nearly four when she vanished from her family's holiday apartment in Praia da Luz in the Algarve on May 3, 2007, as her parents dined with friends nearby.
Despite a massive police investigation and huge publicity worldwide, she has not been found.
The official Portuguese inquiry into Madeleine's disappearance was formally shelved in July 2008, although private detectives employed by the McCanns have continued the search.