The Portuguese holiday resort of Praia da Luz from where Madeleine McCann disappeared three months ago

Police investigating the disappearance of Madeleine McCann are carrying out tests on blood traces found inside her apartment bedroom, it has emerged.

The dramatic discovery was made by British detectives brought in to launch a review of evidence, and led to renewed criticism of an "inept" Portuguese investigation.

The British team used specially-trained sniffer dogs and ultra-violet technology to scan for specks of blood inside the holiday apartment in Praia da Luz where Madeleine disappeared 96 days ago.

Tests will now establish whether the traces are those of the four-year-old.

The review, led by Leicestershire police, also focused this weekend on the home of Robert Murat, and is expected to be the final stage in clearing him of his status as the only formal suspect in the case.

Samples have been sent away for urgent DNA testing and detectives believe the discovery could "change the direction" of the investigation.

A Portuguese police source said: "If the results are positive, this will open up a completely new line of inquiry".

The tiny traces of blood - invisible to the naked eye - were found at a low height on the wall in the bedroom of the McCann holiday apartment at the Ocean Club.

Specially trained cocker spaniel sniffer dogs, which are able to detect blood up to seven years old, located an area of the bedroom in which to search. The windows were blacked out using a tarpaulin and a specialist ultraviolet torch pinpointed the specks of blood.

Police sources played down reports in a Portuguese newspaper that the blood had been identified as Madeleine's.

Previous forensic tests have taken a month to return from the national laboratory.

The potential breakthrough led to immediate criticism of Portuguese police for missing key forensic evidence, and failing to bring in outside expertise earlier in the investigation.

The apartment was searched this weekend even through it had been "released" as a crime scene by Portuguese authorities on June 11, cleaned and rented out to other families.

Former detectives are mystified as to why it has taken more than three months to make the discovery. Mark Williams-Thomas, a former Surrey detective and now a leading child protection expert, said: "I am staggered that it has taken so long.

"The police should have sealed the apartment immediately, on day one and then conducted a thorough forensic examination- this would have taken days and would have involved analysis for fluids and fibres and involved stripping the apartment bare.

"Even if this proves not to be significant, it should have been discovered in the first few days and eliminated. This shows just how inept the Portuguese police were at carrying out the initial forensics tests.

"Just imagine the impact it will have on Gerry and Kate so many months on - to find that something so vital was missed right at the beginning. "It is great that British police have finally carried out a review but you have to ask why has it taken three months for it to happen?

"I called for a review within the first month, especially because of the relative lack of experience of the Portuguese team working on this.

"It’s astounding that it has taken this long to bring in specialist help. It makes a mockery of the Portuguese investigation."

British police dog handlers had offered expertise to Portuguese authorities in May. They were only invited to join a "review of evidence" - which is now commonplace in high profile investigations in Britain - last week.

A two-day "final" search of Mr Murat’s villa - 150 yards away from where Madeleine disappeared - uncovered no new evidence. As many as 10 officers had spent Saturday clearing thick vegetation.

They began digging today and two British police technicians used specialist "Pulsar" scanning equipment to detect whether any earth had been "disturbed" in the past few months.

Mr Murat, 33, has been the only official "arguido", or suspect, since May 10, when he was detained by police. He was questioned for a second time last month but detectives have never found any evidence to arrest him formally.

Members of the Murat family today drove four cars shared by Mr Murat with the family to a police headquarters for them to be searched again by the sniffer dogs.

Mr Murat has welcomed the searches, which he believes will be the last step in proving his innocence.

His mother, Jennifer Murat, 71, insisted: "He is going to be cleared. He is innocent."

It has also emerged that police are also investigating possible links between Madeleine’s case and a suspected child abductor in Switzerland who committed suicide last week.