Kate takes drug tests to prove she wasn't on anti-depressants when Madeleine vanished

The mother-of-three wanted to counter allegations that she needed medication because she was hysterical and unstable around the time Madeleine went missing on May 3.

Leaks from inside the Portuguese police investigation suggested detectives believed that Mrs McCann might have killed her daughter because she could not cope with the "hyperactive" child.

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But results of the toxicology tests on hair samples from Mrs McCann show no evidence of any drugs in her body at any stage in the last eight months.

Hair from her two-year-old twins Sean and Amelie was also tested to show they had not been given sedatives.

The results will be a key element in a dossier of evidence which the McCanns hope will demolish the Portuguese police case against the couple.

Police have refused to reveal what evidence they have against the pair, but they were made official suspects in September after sniffer dogs detected the "scent of death" in their hire car and microscopic traces of blood in their holiday apartment.

Detectives were said to believe that Mr and Mrs McCann, both doctors, had drugged their children to ensure they slept while the couple went out for dinner with their friends.

It was alleged that Madeleine died in an accident in the apartment - possibly from an overdose of sedatives - and the couple then hid her body to avoid accusations of child neglect and to protect their professional reputations, it has been claimed.

Police drafted in criminal psychologists to analyse Mrs McCann's mental state and seized her diary, which allegedly contained entries suggesting she had struggled to cope with looking after her children.

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Detectives were said to have applied to see her medical records to prove she was suffering from clinical depression.

She has always denied the claims, and any suggestion she was involved in Madeleine's disappearance.

The drug tests were carried out by Home Office approved experts in September after the finger of suspicion was first pointed at the couple.

Edward Smethurst, the lawyer co- ordinating their defence team, told the Evening Standard: "Hair grows by about a centimetre a month so if you have eight centimetres of hair, you can test for drugs going back eight months.

"With the appropriate, fully accredited experts, hair samples were taken from Sean, Amelie and Kate and the conclusion was no evidence of sedatives or drugs were found.

"There were various stories circulating that Kate might somehow be unstable and might be depressed, but the tests demonstrate that the kind of drugs she might be on did not show up."

Independent tests were also carried out on the couple's Renault Scenic hire car and found no traces of Madeleine.

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Mr Smethurst said: "We believe it highly unlikely charges will ever be brought against Gerry and Kate. Now we just want to focus efforts on finding Madeleine."

Red tape could delay plans to re-interview Kate and Gerry McCann for up to six months, it was revealed.

Letters requesting fresh interrogations of the couple and their friends - the Tapas Nine - have to go through four different ministries, a Portuguese newspaper reported. They are progressing "at a snail's pace", according to newspaper 24 Horas.

Mr and Mrs McCann have said they are willing to face new interviews if it would help solve the case or dismiss suspicions against them.