The police case against Kate and Gerry McCann for killing their daughter appeared to be crumbling yesterday after a senior prosecutor said that Madeleine’s body must be found to secure a conviction.

The development came as the McCann family said that they expected to spend a year fighting to clear their names, and that they would rather Madeleine were found dead than be left in the “awful limbo” of not knowing her fate.

It is also thought that former military personnel are searching for the girl, who disappeared just before her fourth birthday.

António Cluny, the president of the Portuguese public prosecutors’ service, said that it appeared that there was insufficient evidence that Madeline was killed by her parents. His comments come after an increasing number of reports in Portugal that samples taken from the McCanns’ apartment and hire car are inconclusive. Last week a judge said that police had failed to gather new evidence to justify reinterviewing the couple.

Mr and Mrs McCann have insisted that any samples from Madeleine found in the Renault Scénic hired 25 days after her disappearance could have come from her clothes or belongings that were carried in the car.

Mr Cluny told the 24 Horas newspaper: “Without the little girl’s body everything is extremely complicated. There have been cases where it has been possible to obtain a conviction without there being a victim, but there were confessions.

“One cannot accuse a person of homicide without there being very strong evidence. In the Maddie case there is no confession, and according to what has been made public the evidence gathered up until now keeps all leads open, from abduction to homicide or at least to a simple accident.”

When asked if it would be possible to secure a conviction, he said: “It is highly possible; it is just harder.”

His comments will be a blow to senior officers in the PolÍcia Judiciária (PJ), who have told Portuguese journalists that they have compelling evidence that the McCanns accidentally killed their daughter before disposing of her body.

The PJ is reported to have been ordered by the public prosecutor to find Madeleine’s body or accept that Mr and Mrs McCann will not be charged. A police source told 24 Horas: “It is our great priority . . . Finding the body is fundamental to solving the case.” The source said that the case against the McCanns, from Rothley, Leicestershire, was “hanging by a thread”.

A source close to the McCanns’ legal team said yesterday that they expected to remain arguidos, or formal suspects, for months. Portuguese law allows for the status of formal suspect to be extended for up to a year. The source said: “The mood is that they’re in this for the long haul. They’re assuming that this could go the full year.”

The McCanns fear a “worst-case scenario” of Madeleine’s body never being found, a friend said: “If there is no body found and they are not charged and their arguido status falls away, then they’ll have this hanging over their heads. But that’s unacceptable because they are innocent. Why should they have this stain on their characters? It would be the worst-case scenario because nobody, least of all them, will ever know what’s happened.”

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