The number of police officers dedicated to the Madeleine McCann investigation is to be reduced from 29 to four, Scotland Yard has said.

Operation Grange was launched in 2011 at the request of the Home Secretary, who made additional funding available for British police to support the ongoing Portuguese operation.

The UK probe has cost more than £10 million so far, but has yet to yield any arrests.

Madeleine was three years old when she went missing from her family's holiday apartment in the Algarve on 3 May 2007.

In the years since, police have questioned numerous people - including 11 witnesses in December last year - but no arrests have been made.

ITV News Correspondent Emma Murphy reports:

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.

We still have very definite lines to pursue which is why we are keeping a dedicated team of officers working on the case.

Assistant Commissioner Mark Rowley
An age progression image showing how Madeleine may have looked aged eight. Credit: Teri Blythe/Metropolitan Police

A senior Portuguese police source told ITV News the Metropolitan Police Service didn't tell them they were reducing the number of officers on the inquiry.

The source said Portuguese police see it as a move by the Met to prepare the way for shutting down the inquiry altogether.

The Portuguese force are still co-operating with their British counterparts.

Kate and Gerry McCann thanked the police for their work over the past four years. Credit: PA

Madeleine's parents Kate and Gerry McCann thanked police for their "meticulous and painstaking work that they have carried out over the last four and a half years".

In a statement, they added: "We are reassured that the investigation to find Madeleine has been significantly progressed and the MPS has a much clearer picture of the events in Praia da Luz leading up to Madeleine's abduction in 2007.

"Given that the review phase of the investigation is essentially completed, we fully understand the reasons why the team is being reduced.

"We would also like to thank the Home Office for continuing to support the investigation.

"Whilst we do not know what happened to Madeleine, we remain hopeful that she may still be found given the ongoing lines of enquiry."

Detective Chief Inspector Nicola Wall will continue to head the probe and the Home Office has budgeted £2 million to fund it over the next year.