Anxiety was turning to despair last night for the family of Madeleine McCann as Portuguese police admitted they could give "no firm assurance" the three-year-old was still alive.

Five days after her disappearance from her parents' holiday apartment in the Algarve resort of Praia da Luz, the police announced that Interpol and Europol were now assisting the investigation.

They had said on Saturday that they believed she was both alive and still in Portugal.

But last night Chief Inspector Olegario Sousa of the Central Directorate for the Fight Against Crime (DCCB), Portugal's equivalent of the National Crime Squad, was unable to provide that assurance.

While he insisted all efforts were being made to recover Madeleine, when asked how confident he was she was still alive, he said: "It is very difficult to give you an answer because I have not facts to sustain that the child is alive or not.

"We're searching for the child and until the moment she appears we can say nothing more because we are not magicians.

"All the authorities involved are doing the best efforts to recover the child."

At a chaotic press conference in the Town of Portimao the chief inspector said he could say little about the investigation because of restrictions of Portuguese law.

Earlier, speaking from the resort, Kate and Gerry McCann made an anguished plea to whoever had their daughter not to harm her, and to set her free.

"Please do not scare her, please let us know where to find Madeleine or put her in place of safety and tell somebody where," he mother said. "We beg you to let Madeleine come home."

She added: "Please give our little girl back," repeating it in Portuguese: "Por favor, devolva a nossa menina."