The UK-based investigation into her disappearance could finish in the next few months he was quoted as telling LBC radio.
The news comes as the ninth anniversary of Madeleine’s disappearance approaches, having gone missing from a holiday apartment in Praia da Luz on 3 May, 2007.
According to the BBC, the remaining line of inquiry is centred around a letter asking for assistance sent from UK investigators to the Portuguese Public Prosecution Service in July 2015.
The Metropolitan Police Service (MPS) last October reduced the team working on the disappearance of Madeleine McCann from 29 to just four.
This came after widespread cuts in the UK suffered by law enforcement authorities and also revelations that almost 15 million euros of the British taxpayer’s money had been spent since the launch of the case review in 2011 without any success.
Following the announcement, police sources in Portugal said their side of the investigation was ongoing.
In a statement sent to The Portugal News, the MPS said that the investigation into what happened to Madeleine continues, but with a smaller team of officers.
Detectives investigating the disappearance of Madeleine McCann in May 2007 have been working through material and following investigative inquiries since the Home Office requested a review of the case in May 2011.
“While there remain lines of inquiry to follow, the vast majority of the work by Operation Grange has been completed”, the statement explained.
British police said the investigation team has taken 1,338 statements and collected 1,027 exhibits. Having reviewed all of the documents, 7,154 actions were raised and 560 lines of inquiry identified, and over thirty were made international requests to countries across the world asking for work to be undertaken on behalf of the Met.
Officers have investigated more than 60 persons of interest. A total of 650 sex offenders have also been considered as well as reports of 8,685 potential sightings of Madeleine around the world, the statement said.
The Grange team received on average two hundred emails a week, and following the media appeal in October 2013 across three countries, obtained over 7,000 responses.
Portuguese police sources earlier this year told The Portugal News they remained in the dark as to what might have happened to Madeleine.
They explained that there are “about two or three other cases of missing children in Portugal, but we have a reasonable understanding of who might be responsible for their disappearance. However in the case of Madeleine McCann, we are yet to come up with a satisfactory explanation as to what happened to her”.
Sources added that this was a feature of the case that would keep it alive in Portugal even if the case into her disappearance were to one day be shelved.