Scotland Yard's six-year investigation into Madeleine McCann's disappearance was a poisoned chalice laced with critical errors because of a high level agenda to not interrogate the child's parents, according to a former UK detective.

The explosive revelations were made by retired Metropolitan Police homicide cop Colin Sutton, who at one time was touted as a possible candidate to lead Operation Grange and the search for Maddie, now missing for 10 years.

Operation Grange's narrow remit to focus only on the theory that the four-year-old was abducted from the family's holiday apartment in Portugal was unusual and a "missed opportunity", Sutton told nine.com.au.

In 2010, with planning underway to launch Operation Grange, Sutton received a phone tip off from "a very senior Metropolitan police officer", warning him about the looming investigation and how it would be handled.

The insider told Sutton, who served 30 years with London's Met before retiring in 2011, that the dozens of murder detectives assigned to Operation Grange would be instructed where they could and couldn't look.

"I immediately assumed that what was meant was that the [McCann] family and Tapas 7 [the group of seven friends on holiday with the McCanns] were a no-go area," Sutton said.

Gerry and Kate McCann aboard Sir Philip Green's private jet at Faro airport in Portugal, as they leave for Rome to prepare a meeting with the Pope Benedict XVI on 30 May, 2017. Source: AFP

In May 2011, when Operation Grange was launched, the detective's instincts were proven correct.

The "crucial phrase", as Sutton calls it, in the Operation Grange remit was a line stating the review would be carried out " as if the abduction occurred in the UK ".

That meant Kate and Gerry McCann, despite several concerning inconsistencies in their witness statements, were not to be looked at, Sutton said.

"It was almost this unspoken elephant in the room," he told nine.com.au.

"The rest of [the remit] is really of little consequence after that because that's sort of saying … we are only treating this as an abduction and we are not looking at any other scenario."

Sutton also hit out at Scotland Yard claims that the McCanns, who have always denied any involvement in the disappearance of Madeleine, had been cleared by Portugal's police force, the Policia Judiciaria (PJ).

Portuguese authorities shelved the investigation in 2008, 14 months after Madeleine vanished on May 3, 2007, and in doing so lifted arguidos (formal suspect) status from the McCanns.

"The PJ have never cleared anyone," Sutton said.

Ceasing the investigation "just meant they couldn't find enough evidence to proceed against them. Their view is that the parents are certainly not eliminated".

Madeleine Beth McCann: missing since May 3, 2007. Source: Getty.

Sutton, who led more than 30 successful murder investigations, said it was well-rehearsed, best police practice in cases such as Madeleine McCann to eliminate those closest to the child first.

"Also any kind of investigation of murder or akin to murder the other place you need to eliminate early on is those that last saw the victim alive.

"In this case you've got essentially the same group of people who are both close to the victim and the last to see her alive. I'd always want to start with that.

"I don't understand why that hasn't been done [by Operation Grange], because it would appear to be in everyone's interest."

Earlier this month, Assistant Commissioner of London's Metropolitan Police Mark Rowley denied Scotland Yard had a closed mind to the possibility of Kate and Gerry McCann’s involvement.

"The involvement of the parents, that was dealt with at the time by the original investigation by the Portuguese," Asst Com Rowley said during a media briefing.

"We had a look at all the material and we are happy that was all dealt with and there is no reason whatsoever to reopen that or start rumours that was a line of investigation."

When asked if Kate and Gerry McCann had ever been questioned as potential suspects by Scotland Yard detectives, Asst Com Rowley replied: "No."

Colin Sutton: The Operation Grange remit showed that they were only looking at an abduction as the only possible scenario. Source: Sky News

Sutton said he disagreed with Asst Com Rowley's assessment. He said inconsistencies in some of Kate and Gerry's statements, Kate's 2011 book madeleine and also some of the witness accounts of the Tapas 7 disturbed him.

The Portugal detective who oversaw the original investigation, Goncalo Amaral, wrote a book theorising Maddie had died in apartment 5A, that Kate and Gerry had disposed of the body and the parents had faked their daughter's abduction.

After police found no forensic evidence in the apartment to back up claims of a break in, Gerry's statements to police detailing what doors he and Kate had used while checking on their three sleeping children changed.

Portugal's police also had some doubts over the accuracy of timelines provided by Kate and Gerry, and the Tapas 7, in the critical hours either side of Maddie being reported missing at 10pm.

Specialist cadaver and blood dogs were brought to Praia da Luz from the UK, and signalled hits inside apartment 5A and a hire car rented by the McCanns 25 days after Madeleine disappeared. DNA swabs were taken but ruled inconclusive.

"There was a part of me that always had this hope in the back of my mind that actually there was lots of busy and important covert work going on in the background of Operation Grange, that there was going to be some kind of bombshell announcement," Sutton said.

"I fear that is not going to be the case now. I fear it will just peter out and probably this thing will never get resolved."

It was "entirely possible" that some of Operation Grange's remit was forced upon Scotland Yard by government officials who rubber stamped the multi-million-dollar funding of the investigation, Sutton said.

In March Operation Grange was injected with an additional $150,000 to cover the investigation through to September, 2017.