A British sniffer dog detected the scent of a body inside the apartment of Madeleine McCann, according to Portuguese police.

Olegario Sousa, a spokesman for the Policia Judiciaria, said they are now working on the assumption that the four year old is dead.

Detectives remain convinced despite reports last night that preliminary test results on blood specks found inside the bedroom of the apartment were not those of Madeleine.

Police have played down the significance of the blood traces, saying they were not the “decisive” evidence in the case and that they remained convinced that Madeleine died on the night she disappeared.

Mr Sousa's comments came as Kate and Gerry McCann admitted for the first time that they have discussed returning home to Britain without their daughter.

The four-year-old went missing from a holiday apartment in Praia da Luz 105 days ago and the couple have insisted they could not face leaving Portugal while the search continued.

But in an interview Mrs McCann said: "We know we will be going back and I guess one day we will wake up and it will be right.

"We never thought that we would go before she came back. Now we just don't know. We have the twins to consider.

"I can't imagine how we came out as a family of five and going back as a four."

After exhausting all leads suggesting Madeleine was abducted, police are now working on the theory she suffered an accident or was killed inside the flat.

They have confirmed that the parents are not suspects. Two specialist British sniffer dogs, one with the ability to find small traces of blood and the other a "victim" dog who can detect human remains, were brought in a fortnight ago.

Speaking to a Portuguese TV station, Mr Sousa said that they had traced the scent of a body inside the family apartment where Madeleine disappeared and found specks of blood.

Tests are still ongoing on spots of blood and the inside of the bedroom of the apartment and the McCann family have been told that results will only be confirmed next week.

However, a four-page interim report received by Portuguese police has concluded that the blood is not Madeleine’s. Forensic analysis suggests that there is a 72 per cent chance the sample is that of a white man of “northeast European” origin. The condition of the sample is poor because the wall had been cleaned with detergent.

Mr Sousa also tentatively suggested there could new suspects in the case depending on the results.

He said: "Naturally we could have new arguidos, depending on the result [of DNA tests] there could be a change in the procedural position of those involved in the case or, eventually, there could arise other people, others involved who until now are not known".

Mr Sousa insisted the results would not be "decisive" in the case and there were other signs "giving strength" to the theory that the child is dead.

However the evidence of the sniffer dogs, which were only used three months into the search for Madeleine, has thrown up more questions than answers.

Expert handlers said that dogs can only pick up the scent of a body which has been in situ for more than two hours.

The springer spaniels used can also detect small traces of blood up to seven years old, in which time hundreds of people have used the holiday apartment where they were found.

Earlier this week, Portugal's most senior police officer played down developments. Alípio Ribeiro, national director of the PJ, said that detectives were "far from throwing light on the case".

He added: "Despite the fact that new elements have appeared in the investigation we still do not know where they will lead us."

Mr Sousa said the case "is complex" and "extremely difficult to investigate".

He defended Portuguese police for missing the evidence found by the British sniffer dogs and added that police teams had found a lot of forensics.

"Never has there been collected so much evidence in a crime scene by specialised teams as in this situation," he said.

Police have not ruled out that Madeleine was abducted but have exhausted all leads suggesting so. All 490 guests staying on the same Ocean Club resort have been traced and spoken to.

No one, with the exception of one of the friends holidaying with the McCanns, saw anything suspicious in the area.

Meanwhile, Mr and Mrs McCann have been kept in the dark by police of recent developments and the apparent "shift" in the investigation.

Last week the couple were upset and set back when detectives took a cold, "formal" tone with them at their regular update meeting. Mr McCann said he felt the investigation has gone "back to square one".

In the past 10 days the couple have faced the hardest period since Madeleine disappeared, as Portuguese press turned suspicions on them.

Mrs McCann attacked the local journalists as being "aggressive and intrusive" but said she would not be hounded out of Portugal. Mr McCann said dealing with the intense scrutiny had come as a shock after a quiet period in the case in July.

He said: "The frenzy that built up last week, with the resumed searches and a clear shift in the investigation - if that had happened in the first week or two we would have been prepared for it."