Peter MacLeod, a retired superintendent with Nottingham Police, said the McCanns' claim she was snatched via the narrow apartment window raised "red flags".
He made the accusation in a new podcast detailing Maddie's 2007 disappearance by 9 News journalist Mark Saunokonoko.
Maddie, 3, disappeared on May 3, 2007, having vanished from an apartment in Portugal while her parents Gerry and Kate dined with friends at a tapas restaurant 120m away.
In September of that year, Gerry and Kate, both doctors, were sensationally named as suspects by Portuguese police.
Though they were later cleared and have always denied any involvement in her disappearance, many, including very vocal groups of online trolls, still to this day claim there was a cover-up.
Kate's friend told The Sun Online: "Spurious allegations discussing what might have and might not have happened that night have been made umpteen times.
"Anybody can do a podcast, it doesn’t mean it’s right, and one spouting off about what they did and subsequently said is something quite frankly they will ignore. What would he know?
"Kate and Gerry never said their daughter was smuggled out of the window, they think she was taken out of the door."
Former GP Kate, who celebrates her 51st birthday today, and heart doctor Gerry, 50, from Rothley, Leics, cling onto a glimmer of hope that Maddie could still be alive. She would now be aged 15, nearly 16.
Mr MacLeod cast doubt on the theory an abductor climbed through the apartment window in order to snatch Maddie.
He contrasted descriptions of the window as damaged and "jemmied" with claims made by the bungling Portuguese cops that there was no evidence of forced entry.
He told the podcast producers: "The window is only an absolutely maximum 50cm wide, in reality 46 wide, and it's already a metre off the ground.
"I had a look at that and my shoulders are wider than that little window. So although you could climb in sideways you certainly can't jump in if you are a normal sized person.
"I frankly do not think it's possible. It would be difficult enough to get yourself in. You'd have to grab hold of windows, grab hold of things.
"Then you've got to pick up a child without waking it up without waking the other two children. I do not think it can be done, or let me put it another way, I do not see how anyone could do it."
The new podcast – titled Maddie – is the product of two years investigative work and will draw on analysis from experts and multiple people involved in the case.
Its release comes just as it emerged Netflix has also produced a documentary which Gerry and Kate have refused to be involved with.
It emerged at the weekend they said they want "nothing to do with it" so they avoid jeopardising the ongoing Met Police's investigation into her disappearance.
Netflix had been hoping it would be ready to screen last year but the project reportedly took longer than expected.
It's also thought producers had wanted to create an eight-part series but it may now be a one or two-parter.
Despite that, Netflix are hoping it'll be massive hit like its other successful true crime dramas including Making A Murderer and Abducted in Plain Sight.
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