Allan Scott, a lecturer at the University of Central Lancashire's School of Forensic and Investigative Sciences, warned that any evidence needed to be treated with caution until the full facts were known.

The investigation took a dramatic turn this week when Maddy's parents, Kate and Gerry McCann, who returned to Britain on Sunday, were named as formal suspects in the inquiry - and Portuguese police say they plan to take further action in the coming days.

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.

But Mr Scott, who has worked at UCLan for three years, warned cross-contamination needed to be considered.

He said: "DNA is so sensitive that if you and I met in the street and shook hands and then I went and committed some crime, then I could possibly leave your DNA at the scene.

"It is very difficult to actually say whether these finds are significant or not. There is an information vacuum.

>> Madeleine's parents in heartbreak return home

"Portuguese law prohibits police giving out information which has led to a lot of innuendoes and rumours."

Mr Scott, the former head of Crime Scene Investigations at Merseyside Police, also talked about the use of dogs trained to recognise the smell of dead bodies, which were apparently used by Portuguese police.

He said: "It is not evidence but it is used as an indication. It can usually help to narrow down an area that needs to be searched. But I don't think it can be used as evidence in court.

"Also it isn't clear whether the "blood" found is spots of blood or smears of blood - each are very different because smears would perhaps indicate someone had tried to clear it up. It would put a completely different complexion on things."

Mr Scott, who lives in Southport, was previously called on to help with the painstaking task of gathering vital forensic evidence from the 7/11 London bomb blast sites.