Kate and Gerry McCann have criticised the former police officer in charge of the hunt for their missing daughter after he delayed their libel action against him again.
Mrs McCann said every delay caused by Goncalo Amaral, who sacked his legal team at the last minute, "causes us more pain and distress".
They are suing for libel over claims he made in his book The Truth Of The Lie and had travelled to Portugal to deliver personal statements on how accusations in the book were affecting them.
Mr and Mrs McCann were originally told they could not make statements to the long-running trial at Lisbon's Palace of Justice, but the decision was overturned after an appeal by their lawyer.
Speaking outside the court in Lisbon, Mr McCann said: "We're exasperated that the hearing has been cancelled once again at Mr Amaral's request.
"This is the fourth time this has happened and we've travelled to Portugal.
"The legal case has been running now for over five years and we want to get justice for Madeleine.
"Today is a blatant and cynical attempt to wear us down and it's Madeleine who is suffering.
"We're not going to give up - we're going to keep going."
Clearly angry and upset, Mrs McCann said: "We need to make it clear to people: we took on this case because of the pain and distress that Mr Amaral has brought to us and our children.
"And every time he postpones the case like this it brings us more pain and distress.
"Every time we come here we have to make arrangements for our children to be looked after, we have to book flights, we have to book hotels, we have to take time off work to come here.
"And Mr Amaral handed that letter in apparently around nine o'clock this morning. That letter could have been handed in before we left the country.
"And this has happened about four times. As Gerry said, this can't be seen as anything but blatant and cynical. We just want justice. This is not fair."
Mr McCann said the couple would return, with the next hearing set for July 8.
Madeleine, who was then nearly four, disappeared from her family's holiday apartment in Praia da Luz in the Algarve on May 3 2007 as her parents dined at a nearby restaurant with friends.
Her parents say that claims in Mr Amaral's 2008 book, including suggestions that they hid Madeleine's body after she died in an accident and faked an abduction, damaged the hunt for their daughter and exacerbated their anguish.
If successful the family stand to gain around £1 million in damages.
The McCanns' return to Portugal comes after they said last week that the fact police found no evidence relating to their missing daughter during recent searches in Praia da Luz had reinforced their belief that she could still be alive.
Mrs McCann said Mr Amaral was trying to deny Madeleine the "human right" to be looked for and found, adding that the detective was "not the victim in this".
When asked whether he thought Madeleine was alive, Mr McCann: "What is clear is that the searches haven't found any evidence to show that Madeleine has been seriously injured or is dead. As far as we are concerned there is a good chance she is still alive and we have to keep searching for her - that's why we are here."
The latest delay in the case comes as Mr Amaral made more outlandish claims today about Madeleine's disappearance in an interview with Portuguese newspaper Correio da Manha and its sister TV channel CMTV.
The McCanns' lawyer Isabel Duarte today said that the next hearing is due to take place on July 8, providing Mr Amaral does not revoke his new lawyer's powers again, and said: "No-one imagined that this could happen."
She said delays meant that the book could continue making "severe damages" against the McCanns.
Asked if the latest move by the former detective was a strategic attempt to delay the trial, she said it was "clear" that if someone wanted to remove their lawyer's powers, they would substitute him in the same day.
Mr McCann said Madeleine "is completely innocent in all of this, this is what it's about, an innocent child who is missing".
When asked what he would say to Mr Amaral, Mr McCann said curtly: "We are not speaking to Mr Amaral."