Madeleine investigation grinds to a halt as police are barred from asking suspects 100 new questions

Detectives said a 'root and branch' review of the six-month inquiry had 'confirmed suspicions' but failed to uncover any new clues which could solve the disappearance.

They have drawn up a list of questions they want to put to Kate and Gerry McCann but they will not be allowed to do so unless they convince a public prosecutor that they have a case against them.

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They also want to interrogate the other members of the so-called Tapas Nine, and to quiz relatives about the couple and their relationship with their children.

But the Portuguese public prosecutor has said he will not authorise any new interrogations without seeing stronger evidence in the case.

Police have privately admitted that it would take 'a miracle' for them to build a better case against the couple, although they still hope there could be a forensics breakthrough in the investigation.

A source told the Portuguese newspaper 24 Horas that detectives were waiting for further test results from the Forensics Science Service in Birmingham.

He said: "The tests were requested more than three months ago but the results have not yet arrived. These tests are fundamental to direct the investigation.

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"The letter of appeal (to the British authorities) is concluded. It contains over 100 questions which will be put to Kate and Gerry McCann, their family members and their friends.

"The purpose is to confirm the testimonies produced at the time the little girl disappeared."

Preliminary results from the FSS proved inconclusive, as did forensic tests carried out in Portugal.

An FSS spokeswoman said tests were ongoing, but a Portuguese police laboratory source revealed growing frustrations within the investigation about the lengthy wait for results.

He said: "The truth is that those tests - despite being complex - are carried out in 90 days maximum. One cannot understand the English delay."

Detectives have prepared letters of appeal to the British authorities, asking for British police to re-interview Mr and Mrs McCann, both aged 39, and the seven friends with whom they were on holiday.

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They also want the couple's laptops seized, and some personal items including Kate's bible and diary. But the public prosecutor, Jose Magalhaes e Meneses has refused to authorise the moves without seeing stronger evidence.

A police source told 24 Horas: "The new analysis of the evidence, carried out by the team which took charge of the case, has confirmed all the suspicions that exist but nothing more.

"There is no new data. New operations were carried out, other people who had not been interviewed at the time were interrogated. But nothing new came of it.

"If a body does not turn up or unless somebody comes forward with a credible lead we continue increasingly without any chance of closing the case."

Mr and Mrs McCann have provided police with a list of 25 witnesses they would like to see interviewed, including several relatives, in the hope they would give detectives a better picture of their family relationships.

A friend said they had heard nothing more from detectives since sending them the list last month, and admitted that frustrations were growing.

He said: "Of course there are frustrations this is still unresolved.

"There are frustrations amongst Kate and Gerry's friends, that's for sure.

"They want to help them in any way they can but are barred from talking because of Portugal's strict privacy laws."

Meanwhile it emerged yesterday that police suspected the McCanns as early as July but did not act on their suspicions as they feared they might leave Portugal.

A source told Correio da Manha: "They feared the two would stop co-operating, which is exactly what happened when they were made arguidos."

Publicly the police insisted the parents, of Rothley, Leicestershire, were not suspects, but then named them as arguidos in September.

McCann family spokesman Clarence Mitchell said: "Once again, we will not comment on speculation in the Portuguese media.

"Kate and Gerry have nothing to hide and we hope they will be cleared as soon as possible."