Madeleine police have only 17 cells of DNA as they admit: 'It's mission impossible'

Portuguese experts have admitted that they face an "impossible mission" to solve Madeleine McCann's disappearance through DNA tests because the forensic evidence is so poor.

They said tests at the Forensic Science Service in the UK were "unlikely" ever to get conclusive results because the samples were too small, containing fewer than 17 microscopic human cells.

The samples, which could have been left by Madeleine's finger brushing against a surface, are also badly degraded and contaminated.

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Under suspicion: Kate and Gerry McCann insist they have nothing to do with their daughter's disappearance

A source told the Portuguese newspaper 24 Horas: "It is highly unlikely they will find a DNA connection."

But British scientists said the FSS laboratory in Birmingham could still crack the case.

Expert Dr Ronald Denny said: "It's not a hopeless task. If they have 17 cells they can carry it off.

"It is a minute sample - something which 20 years ago, or even ten years ago, could not have been worked on."

Portuguese detectives had hoped samples from Kate and Gerry McCann's holiday apartment and hire car would prove their theory that Madeleine died accidentally in the apartment and that her body was transported in the hired Renault Scenic.

Portuguese detectives met experts from the FSS last week.

Four months of testing hair, blood and skin cells have so far proved inconclusive but the laboratory is a world pioneer in low copy number analysis - a technique used to produce DNA profiles from microscopic samples.

The technique is so effective it can get a DNA profile from a single cell, taken from a fingerprint, a bloodstain or tiny trace of liquid.

It allows scientists to "grow" extra genetic material from a tiny sample until it is suitable for repeated testing, but it takes several weeks to produce a definitive result.

The technique helped British police investigating the "Trophy Rapist" attacks around the M25 to identify Antoni Imiela as their prime suspect.

Dr Denny, an independent forensics expert based in Kent, said: "A cell is negligible - a smidgen - you can just about see it under a microscope.

"It's the sort of sample someone would leave behind with the touch of a finger.

"But when placed with certain biochemicals a small amount of DNA grows to a size that can be analysed.

"A person, or their possessions, have to be present for the cells to come off."

The McCanns, both 39, have insisted that Madeleine's DNA could have been in the Renault Scenic - hired 25 days after she went missing - because they used the car to transport her toys and clothes.

They have always denied any involvement in her disappearance.

Samples were also taken from the home of the other named suspect in the case, Robert Murat.

Mr Murat, 34, has always denied he was responsible for Madeleine's disappearance from her bedroom in the family's holiday apartment in Praia da Luz on May 3.

Yesterday marked seven months since Madeleine vanished.

Detectives are preparing to fly to Britain to interview her parents and their friends again.

A police source told 24 Horas they might never be able to solve the case if the McCanns and their friends did not co-operate.

"If everybody refuses to make statements, which is perfectly possible, we will be left with our hands tied.

"This is a case which could hang by a thread for years and we may never be able to prove what actually happened to that child."

The McCanns' spokesman, Clarence Mitchell, said the couple a n d their friends - the so-called Tapas Nine - were happy to be re-interviewed in Britain if it would help the investigation.