Detectives leading the investigation into the disappearance of Madeleine McCann are focusing on the theory she suffered an accidental death in the apartment on the night she disappeared, it has emerged.

Senior police officers in the case, including Guilhermino Encarnação and Goncalo Amaral, are convinced that the four year old is dead, and have exhausted all leads suggesting that she was abducted.

It is now understood that the direction of the inquiry changed a month ago, when a key piece of evidence emerged, which has not been disclosed.

A senior police source said: "They say that abduction is no longer the main lead and that accidental death is the strongest theory they are working on.

"They are awaiting test results on forensics from inside the apartment but even without them, they say they have another lead which points to Madeleine's death.

"It could be a murder or it could be an accident, and at the moment they are leaning towards the latter.

"The apartment is the key – all the answers lie there, they say – but they are far from resolving what exactly happened and why the body disappeared."

Police continue to insist that Mr and Mrs McCann are not suspects, and that there is "no logic" in reports smearing their friends as potentially being involved.

Detectives will not elaborate on what evidence they have to support the theory, however it is understood that the new lead emerged in mid July.

Police had released the apartment as a crime scene on June 11. It was cleaned and rented out to another family.

But in mid July they returned and then later called in expert help from British sniffer dogs, who detected new samples, which are still being tested. The apartment now lies empty again and has been locked by police.

With the shift in emphasis, detectives have told Mr and Mrs McCann to abandon plans to leave Portugal amid new hopes of a breakthrough.

The couple last week admitted for the first time they have considered a return to Britain, more than three months after their daughter disappeared in Praia da Luz.

But they have still not formed any "concrete plans" to return and have repeated that the thought of returning home without Madeleine remains unbearable.

Mr McCann said that while he still clings to the hope his daughter is alive, he realised that to find her now would be almost unprecedented.

He said: "We have done a lot of research into missing children and the percentage of minors who have ever reappeared is very small.

"Obviously a very long time has passed, but we will do everything we can to make sure Madeleine does not become a statistic.

"We are going to keep looking for our daughter, because we need to find her.

"It’s possible Madeleine is dead, but is also possible she is alive."