The parents of Madeleine McCann have signed a deal to write a book about their daughter's disappearance.
Publisher Transworld has bought the rights to their story of how Madeleine disappeared from a holiday apartment in Portugal and their efforts to find her.
Madeleine, of Rothley, Leicestershire, was nearly four when she vanished in Praia da Luz, Algarve, in May 2007.
All proceeds from the book will go to the fund to look for Madeleine, which has been at risk of running out.
The official Portuguese inquiry into Madeleine's disappearance formally ceased in July 2008, but private detectives employed by the McCanns have continued the search for the missing child.
The book, entitled Madeleine, will be published on April 28 next year to coincide with the fourth anniversary of her disappearance.
Kate 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.
"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."
'Piece of jigsaw'
Mr McCann, also 42, added the couple were hopeful the 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," he said.
Transworld has acquired the rights to publish the book in the UK, Commonwealth countries and Europe.
It is being written by the McCanns themselves and is currently part-complete.
It will not be the only book about the case. Last month, a court in Lisbon overturned a ban on a book by former police detective Goncalo Amaral. In Maddie: The Truth about the Lie, which was originally published in July 2008, he questioned the couple's accounts of their daughter's disappearance.
Last year a judge had granted an injunction banning further sale or publication of the book. The McCanns also pursued a libel action against Mr Amaral.