Portuguese police were furious when they were accused of messing up the gathering of crucial DNA samples which they hoped would implicate Madeleine McCann’s parents in her disappearance, it has been claimed.

A row erupted after senior officers in the investigation were said to have been called “amateurish” by officials at the Forensic Science Service (FSS).

The Portuguese team flew to Britain a week ago apparently expecting to be told that DNA taken from Gerry and Kate McCann, the family’s holiday flat and hire car would heavily implicate them in their four-year-old daughter’s disappearance.

With no other evidence, they were thought to be pinning their hopes of charging them on the DNA results.

But angry scenes broke out when they were told shortly into the meeting that the tests were inconclusive, partly because the samples were so small.

A source close to the investigation said: “At the meeting there was a row over the relative failure to collect evidence and the correct manner of assessing it.

“The Portuguese police wanted to know why the results they were anticipating did not materialise.

“The FSS made it clear that things could have been protected better.

"It does not mean there was a lot of evidence waiting to be found but the evidence that was collected was done in a very amateurish way.

“The Portuguese took this badly and there was a big debate. They were unhappy and thought it was implicit criticism of the way they had collected the evidence.

“It went down badly and there was a heated exchange of views between the two groups.”

Despite the inconclusive nature of the tests so far, the FSS are continuing to probe further with the samples in the hope of deciding whether they are relevant one way or the other.

The groups met a week ago to discuss the results of DNA taken from the McCanns’ holiday apartment in Praia da Luz and their hire car.

The samples reportedly include microscopic spots of blood from the Ocean Club apartment where Madeleine went missing on May 3 and “bodily fluids” and hair from the Renault Scenic her parents hired 25 days later.

The Portuguese police will this week send a letter to British authorities to notify them of their intention to come to Britain in the next fortnight to re-interview the McCanns, from Rothley, Leics, and the friends who were with them on their holiday.

The McCanns, who remain official suspects in Madeleine’s disappearance, are preparing for a possible return to Portugal should officers want to question them there or take part in an identity parade.

Clarence Mitchell, their spokesman, said: “They have always claimed they would be happy to return if required to help in any way that would assist the police and lead to them being eliminated as suspects.”

Foreign officers would not be allowed to question directly the McCanns and their friends – known as the “Tapas Nine” – but would be present while British detectives did so and would provide a list of questions they want answered.

With little DNA evidence to go on apparently, police will focus on perceived inconsistencies with the group’s official statements to help them crack the case.