The parents of Madeleine McCann have contacted the lawyers of a man charged with murder who successfully challenged sniffer dog evidence. His lawyers claimed it was unreliable and persuaded a judge in the US to throw out prosecution claims that the dogs had detected the smell of a corpse.

Kate and Gerry McCann hope that the case could help them to prove their own innocence.

Two British sniffer dogs, one capable of detecting blood and human remains, were taken to Portugal in early August to help in the investigation. The dog picked up a “scent of death” on numerous items, including Mrs McCann’s clothes and her daughter’s favourite soft toy.

During police interviews the McCanns, both 39, were repeatedly shown a video of the animal “going crazy” when it approached their hired Renault Scenic car.

Mrs McCann could not explain it, but the scent of bodies remains detectable to the springer spaniel “cadaver dogs” for years and her legal team concluded that the scent could have come from her contact with corpses during her work as a doctor.

Portuguese police believe that the couple may have killed their child accidentally and then disposed of the body using a car they hired 25 days later. Although the McCanns do not know the full details of the Portuguese prosecutor’s case against them, they are concerned that it may rest on the dog’s reaction.

Now their lawyers have requested the case files from the ongoing murder trial of Eugene Zapata in Madison, Wisconsin. His estranged wife, Jeanette, a 37-year-old flight instructor, vanished in October 1976 after taking her children to school. Her body has never been found.

Detectives suspected that Mr Zapata killed her but did not have enough evidence to go to court. Mr Zapata, 68, was charged with murder last year after sniffer dogs were brought in. They allegedly detected the scent of human remains in a basement at the former family home. But Dane County Judge Patrick Fiedler ruled that the evidence was inadmissible, saying that the dogs were unreliable. He quoted analysis of the three dogs’ performance record which showed that they were, respectively, incorrect 78 per cent, 71 per cent and 62 per cent of the time.

The judge told the court: “The state has failed to convince me that it’s any more reliable than the flip of a coin.” The jury is considering its verdict.

A source close to the McCanns’ legal team said: “The court papers, giving the legal submissions, are on their way to the McCann team for consideration. At the moment there are no formal charges and therefore there is no formal allegation against which the McCann team can work. We are having to work a little bit in the dark.

“But given that we understand the central plank of what the police are alleging involves sniffer dogs – albeit British ones, which are said to be particularly good – this is important and relevant, and will be raised with the police and brought to the judge’s attention.”

British millionaires are being sought to fund the McCanns’ legal battle with the Portuguese authorities to clear their names. Sir Richard Branson has donated $200,000 to a fighting fund. A spokesman for the multi-billionaire said: “When the McCanns said under no circumstances would they touch the Find Madeleine Fund and mentioned they would sell their house, Richard felt he had to do something. He is a father and there is a missing child out there.”

Father Haynes Hubbard, the Anglican priest who became close friends with the McCanns when they stayed on in Praia da Luz, said yesterday: “That’s absolutely wonderful. If I had £100,000 I would give it to Kate and Gerry, too.”

Clarence Mitchell will be employed by one of the McCanns’ wealthy supporters to act as the couple’s spokesman. He will resign as head of the media monitoring unit of the Cabinet Office today and take up his new post tomorrow. A friend said he had kept in touch with the couple after handling the case while still working for the Government.