Police have completed the latest search effort in the16-year-old hunt for missing British girl Madeleine McCann after collecting unspecified samples by a reservoir in Portugal, as a German prosecutor played down hopes of an imminent breakthrough.

Portuguese police said in a statement the three-day operation requested by Germany was over and that the collected material would be handed over to German authorities after "safeguarding the interests of the investigation still ongoing in Portugal".

Police would not say whether any useful clues were found during the search that involved sniffer dogs, use of a tractor-based tree-cutter and investigators raking the cleared ground in a few small areas.

A source close to the investigation said there was nothing tangible to report.

A large section of the Barragem do Arade reservoir had been cordoned off since Tuesday morning, around 50km from the Algarve resort where Madeleine went missing in May 2007.

German authorities, who have named a suspect in the case, have been helping Portuguese colleagues comb the remote area inland from Praia da Luz where Madeleine - then aged three - disappeared during a family holiday.

"Of course there is a certain expectation, but it is not high," prosecutor Christian Wolters said.

It was important to show that authorities were investigating the case, he added.

German prosecutors last year named Christian Brueckner an official suspect in Madeleine's disappearance.

The convicted child abuser and drug dealer is behind bars in Germany for raping a 72-year-old woman in the same part of the Algarve.

Brueckner, 45, has reportedly denied any involvement in the disappearance. No body has been found.

British police who assisted their Portuguese and German counterparts at the reservoir had left by early this afternoon, followed byGerman investigators who packed up their tents at a camp on a hill.

Madeleine was three years old when she disappeared

Authorities have not revealed what triggered the latest search operation, but Mr Wolters had said they were acting on the basis of "certain tips".

He told German public broadcaster NDR the new information had not come from the suspect and they did not have a confession or "any indication from the suspect of where it would make sense to search".

The Sun reported that investigators previously found photos and video of Brueckner at the reservoir.

Portuguese lawyer Marcos Aragao Correia previously claimed that criminal contacts had told him that Madeleine's body was in the reservoir, and in 2008 he raised funds for unsuccessful private searches of the water.

Additional reporting PA