Four years after British toddler Madeleine McCann vanished from a holiday apartment in Portugal, her mother describes in a book how she believes the abductor was tipped off about the family's movements.

Kate and Gerry McCann walk after speaking about the disappearance of their daughter Madeleine at a news conference in Quorn, central England, November 2, 2010. Photograph by: REUTERS/Darren Staples Credit: REUTERS

Kate McCann reveals she thinks the case was sparked by a comment in a restaurant reservation book at the Algarve resort of Praia da Luz, saying where the parents would be eating while the children slept in the apartment nearby.

In Madeleine, the 43-year-old says that despite her composed appearance in the days following the disappearance, she was wracked by grief and contemplated committing suicide by drowning in the sea.

Madeleine went missing from the apartment on May 3, 2007, a few days before her fourth birthday, as her parents and a group of friends they were spending the holiday with dined at the restaurant in the holiday complex.

The case triggered massive interest in Europe but despite a huge investigation, she has never been found.

Portuguese police wound up their investigation after 14 months, but her parents say they will not stop their search until they have found their daughter.

Proceeds from the book, to be published in Britain on Madeleine's eighth birthday on Thursday, are designed to replenish the dwindling campaign fund to find her -- a fund that reportedly once stood at ?2 million ($3.3 million, 2.3 million euros).

Kate and her husband Gerry also hope the new details of the case will jog the memory of anyone who may have information about their daughter's disappearance.

"We hope and pray that it will bring us the result we long for and that not only the book but this whole ordeal and heartache will be behind us before too much longer," Kate McCann said.

In the book, she describes her shock at discovering a year after Madeleine vanished that an abductor could have found out the family's movements from the restaurant reservation book.

"When I was combing through the Portuguese police files... I discovered that the receptionist's note requesting our block booking was written in a staff message book, which sat on a desk at the pool reception for most of the day.

"To my horror, I saw that, no doubt in all innocence, the receptionist had added that we wanted to eat close to our apartments as we were leaving our young children alone there and checking on them intermittently.

"This book was by definition accessible to all staff and, albeit unintentionally, probably to guests and visitors, too."

The McCanns, who are both doctors, have been criticised for their decision to leave their children -- Madeleine and her then baby brother and sister -- in the apartment while they dined and take it in turns with their friends to check on them.

"This decision has naturally been questioned time and again, not least by us," Kate writes.

"It goes without saying that we now bitterly regret it, and will do so until the end of our days.

"But it is easy to be wise after the event," she writes, adding that if she had had any doubts about the children's safety, she would have hired a babysitter.

She said that in the days following her daughter's disappearance, "I had an overwhelming urge to swim out across the ocean, as hard and as fast as I could... until I was so far out and so exhausted I could just allow the water to pull me under and relieve me of this torment."

Five months after Madeleine's disappearance, Kate and Gerry McCann were made formal suspects in the case by Portuguese police but were later cleared.

They later won a ?550,000 libel payout from a British newspaper group which had doubted their innocence.