Kate and Gerry McCann have flown to Portugal to speak to their lawyers about that matter and to get an update on the search for her.

It is the first time Mrs McCann has returned to the country since being quizzed by police over her daughter's disappearance.

The couple have already successfully taken out an injunction against further publication of the book by ex-Portuguese police officer Goncalo Amaral.

Now the McCanns could launch a defamation action against Mr Amaral and his publishers, and are believed to be looking for ?1.2m in compensation.

In The Truth Of The Lie, Mr Amaral claims that Madeleine is dead and questions the McCanns' account that she was taken while they were eating with friends nearby.

Sky News crime correspondent Martin Brunt said the couple thought the book was "very hostile" and "deterred potential witnesses in the case from coming forward".

Mr Amaral was involved in the initial disappearance investigation but was later taken off the case.

Madeleine was three when she went missing in the resort of Praia da Luz in May 2007.

The McCanns, both 41, flew in for a series of meetings with their lawyers and other advisers in Lisbon, spokesman Clarence Mitchell said.

"They are planning to be there for one day. They are not going to Praia da Luz," the spokesman went on.

Mr Mitchell added: "It is Kate's first visit back to Portugal since 2007 and it is difficult.

"It's something that has always been daunting for her, but she knew she would go back at some stage - and she's always said she'll do anything necessary to keep the search for Madeleine alive.

"If it means a trip to speak to the Portuguese media, then so be it. But she's always said she'll do anything to keep the inquiry alive."

Earlier this month, a Portuguese judge banned further sale or publication of Mr Amaral's book.

The injunction also banned Mr Amaral from repeating his claims about Madeleine or her parents, who are from Rothley, Leicestershire.

At one point, Portuguese police made Mr and Mrs McCann "arguidos", or formal suspects, in their daughter's disappearance.

They were questioned by detectives but their arguido status was later lifted.