Madeleine McCann
British police are searching the sewer network for clues.Reuters

Police investigating the disappearance of Madeleine McCann have been searching in for clues drains in the area of scrubland in the resort of Praia da Luz.

On the fourth day of the search, at least three manhole covers were lifted and miniature cameras passed through underground pipes into the sewer network.

The sewer system was installed as part of failed plans to build a hotel on the site before Madeleine disappeared in 2007, but after a planning dispute, it remains as wasteland.

The area, which is 5 minutes walk from the Ocean Club apartment where Madeleine went missing in May 2007, has been sealed off, as police use sniffer dogs, digging and radar equipment to search the location.

Tents have been erected at several locations, including over a hole concealed by corrugated iron.

Forensics officers used large sieves to sift through soil for evidence as Detective Chief Inspector Andy Redwood, who is leading the investigation, supervised the search.

While police have not disclosed the intelligence that led them to search the scrubland or details of any findings, Madeleine's parents, Kate and Gerry McCann, today said they were 'encouraged by the progress' of the search.

Writing on the official Find Madeleine Facebook page, the couple said: "We are kept updated on the on-going work in Portugal and are encouraged by the progress.

"Thank you for continuing to stand by us and supporting our efforts to get Madeleine home."

‎Mr and Mrs McCann, who have not travelled to Praia da Luz, were previously said to have been bracing themselves for 'significant news'.

Meanwhile, sniffer dogs used to hunt for murdered schoolgirl April Jones were returned to the site hunting for evidence.

The renewed search is a joint effort between police in the United Kingdom and Portugal and follows a Metropolitan Police review of the seven-year-old case.

Officers have permission to search the area only until Friday (June 6) but have asked authorities in Portugal for an extra week.

Madeleine was three years old when she was abducted from the Algarve resort while on a family holiday with her parents, Kate and Gerry McCann.