Startling new theory emerges about Madeleine McCann's disappearance from ex-detective.

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An investigative journalist and former detective has appeared on British morning television to share a new theory about the disappearance of Madeleine McCann.

Mark Williams-Thomas appeared on This Morning claiming the three-year-old left the apartment the family was staying in to go and find her parents on the night she went missing in May 2007.

The family were on a family holiday in Portugal when Madeleine's parents, Kate and Gerry McCann, left her and her younger twin siblings asleep in the apartment while they dined at a restaurant 50 metres away.

The parents took turns to check on the children at regular intervals before Maddie was noticed missing at around 10pm.

Williams-Thomas said that Maddie had asked her parents on the morning of her disappearance where they were the previous night.

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"I think Maddie was aware they were in the tapas bar in the resort," he told the show's hosts, Phillip Schofield and Holly Willoughby.

"In order to get to the bar you have to come out of the premises, walk on a public road and go back in again.

"The concern I have, I believe she woke up and went looking for them, she left the apartment and came out, we already know the patio door was insecure."

Williams-Thomas also referenced the McCann's ongoing legal battle to stop the publication of a book written by former Portugeuse detective Goncalo Amaral, which claims the pair covered up their daughter's death by staging an elaborate abduction.

"The sadness of all of this is that the real focus is Maddie and should always be Maddie but the legal process between Goncalo and the McCanns detracts from what is going on," Williams-Thomas said.

He said Kate and Gerry still maintain that their daughter - who would now be 14 - is alive and "no physical harm has come to her".