Officers searching an area of scrubland in Praia da Luz, where Madeleine McCann disappeared in 2007, have begun digging.

It comes after British police brought in sniffer dogs on Tuesday to help search the area which is located a five-minute walk from the holiday apartment where she disappeared.

The victim detection dogs, Tito and Muzzy, took part in the 2012 search for murdered schoolgirl April Jones.

At least 30 officers arrived early on Tuesday morning for the second day of the search. Sources understand a team of specialist forensic archaeologists are also working on the site.

They used blue and white police tape to divide the land into sectors for the search. Two sniffer dogs and their handlers joined police in scouring the large cordoned off area while two Portuguese police horses patrolled the perimeter.

Three police tents set up on the site are thought to be where the teams are storing kit and taking drinks breaks rather than covering any significant parts of the site.

The teams spent Monday securing and surveying the site and will soon bring in ground-penetrating radar equipment to try to spot any unusual patterns in the terrain.

Radar expert Rom Gostomski, from London-based firm Sandberg, said "The radar uses electro-magnetic waves that are fired into the sub-surface at a pretty rapid rate and we measure what comes back from those signals".

The hot and predominantly dry climate on the Algarve, where temperatures regularly top 30 degrees Celsius, mean the ground will be far drier than in northern Europe.

"GPR works best in dry conditions - it doesn't like wet, saturated ground because of the high dissolved mineral content" Mr. Gostomski explained.

"In a dry country like Portugal you'll generally get much, much better data and penetrate much deeper".

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There are different types of radar equipment the police could use, but generally the devices allow officers to monitor the ground at least two metres below the surface.

Mr. Gostomski added "If you have got a buried object that is sufficiently different, with different properties from the surrounding area, it will show up very, very clearly; whereas if you have got a gradual change it can be difficult to detect".

Scotland Yard say they will not provide a "running commentary" on the searches.

Former Scotland Yard detective John O'Connor told Newstalk Breakfast earlier he thinks at least two people were involved in Madeline's disappearance.