A former top Scotland Yard homicide detective once tipped to lead the UK investigation into Madeleine McCann's disappearance has claimed he received a phone call warning him not to take the job because he would be told where police could and couldn't look.

Speaking on Maddie , nine.com.au's podcast investigation into Madeleine's disappearance, retired detective Colin Sutton said London Metropolitan Police's Operation Grange was "hobbled" from the start and so its chances of finding out what happened to the missing girl were diminished.

The revelations from Mr Sutton come on the 12th anniversary of Madeleine's disappearance, after she vanished on May 3, 2007 , while on holiday with her family in Portugal.

"I was privately told by a senior officer that it was going to be an investigation where you were told what things you could and couldn't look at," Mr Sutton said.

"The remit and the focus of Operation Grange has been so narrow that it probably was hobbled from the beginning and didn't really have a chance at succeeding."

Mr Sutton solved more than 30 murders for London Metropolitan Police. He famously caught English serial killer Levi Bellfield and brought Britain's worst rapist, Delroy Grant, known as The Nightstalker, to justice.

Since its launch in 2011, Operation Grange has cost British taxpayers more than $20m but Scotland Yard detectives have made no arrests in the search for Madeleine.

Gerry and Kate McCann, the parents of the missing 3-year-old girl Madeleine McCann, walk with their twins outside their resort apartment on May 11 2007, in Praia da Luz, southern Portugal. (AAP)

Over the years Operation Grange has given away little public information about its activities. In the podcast, Mr Sutton looks at public statements made by senior ranking figures inside London Metropolitan Police which appeared to indicate Operation Grange was only investigating Madeleine being abducted.

Jim Gamble, who was involved in the original Madeleine investigation, rejected criticism of Operation Grange. He said the investigation was "well equipped" to solve the mystery and that he had complete faith in detectives on the case.

"I don't think [the investigation] has been a failure. Let's see what comes from it all. There will be evidence collected that at some time, some stage in the future may be able to be help hold someone to account for this," Mr Gamble told nine.com.au .

The former lead Portuguese detective on the case, Goncalo Amaral, and another former Scotland Yard officer both question aspects of Operation Grange in episode nine of Maddie .

Mr Amaral alleged Operation Grange appeared to be a "clean up" job.

He also claimed UK police were preparing to likely to target a German paedophile as the man who abducted and possibly killed Madeleine, but believed that child sex offender, currently in prison, would be a “scapegoat”.

The German, a known sex offender, had lived in the Algarve. Portuguese police investigated him but found nothing suspicious, Mr Amaral told nine.com.au .

"[Operation Grange detectives] are preparing the end of the investigation, with a German paedophile who is in prison right now," Mr Amaral said.

In the forthcoming episode 10 of Maddie , Mr Amaral talks more about his theory about what happened to Madeleine. The podcast has been downloaded more than 1.5 million times, and has reached No.1 on the UK, Australia and New Zealand iTunes charts.

The head of Operation Grange, DCI Nicola Wall, has so far yet to acknowledge an offer from one of the world's top DNA scientists – first revealed in the Maddie podcast - to solve 18 vital DNA samples at no cost and in less than a week.

Episode 10 of Maddie will reveal new developments in the podcast's pursuit of DNA data that Dr Perlin is seeking.

Mr Sutton has wondered why Operation Grange appeared to be ignoring the offer from American forensic expert Dr Perlin to solve DNA evidence from the cold case.

He said Scotland Yard could be sitting on "a real game changer".

Mr Gamble has also said he believes a DNA breakthrough could breathe new life into the case.