FORENSIC evidence received by Portuguese police from scientists in the UK appears to be behind the recent dramatic twists in the Madeleine McCann investigation.

But experts in the science warned today that any evidence needed to be treated with caution until the full facts of the case were known.

The samples of DNA now at the centre of the investigation - and the subject of questions put to the McCanns - were taken from two sources, according to reports.

One is thought to be the Algarve apartment from where the four-year old vanished, and the other is believed to be a hire car used by the McCanns 25 days after her disappearance.

If DNA with a close match to Madeleine was found in both locations, then it would have raised serious questions.

But Alan Baker, a forensic scientist who has given expert evidence on the subject in court cases, said several important factors needed to be considered first.

He said the type of sample - whether actually blood or just a smear - was vital in interpreting any potential match.

“If they have found a hair follicle or a trace of blood at the scene then the implications could be immense but if it is only a smear then there are all sorts of issues involved”, he said.

“If they have just found traces of DNA in the vehicle or the flat then that offers up all sorts of explanations, and you have to look at how it could have got there.”

He said the science of matching DNA profiles was also made very difficult by the fact that members of the same family were involved.

If the match was less than perfect, as some newspaper reports have suggested, then it becomes more likely that any DNA is not Madeleine’s, but maybe belongs to her siblings or parents.

“The interesting thing about this is that in most crime scenes, the individuals in question are not related so the DNA is completely different,” Mr Baker said.

“But in this case you have got members of the McCann family involved and they would have shared DNA with their offspring.”

He also pointed out that any cross-contamination of DNA had to be taken on board as a possible explanation if it was Madeleine’s profile in the car or flat.

Mr Baker also urged caution over how the samples were actually obtained.

He said he was confident that any DNA testing by Britain’s Forensic Science Service (FSS) would have been reliable, but added they had no control over how any samples were collected.

He said: “FSS are world leaders in this sort of work and they would not have put their name to anything that they are not happy with.

“But the sample is only as good as the person that took it so if you have sloppy police work then you have all sorts of issues to do with that.

“If the crime scene was in Britain then I would be 100 per cent confident in the integrity of those samples but if they were taken by poorly-trained people then there are all sorts of issues.”