Madeleine: Tourists asked for DNA

The move is said to have come after detectives uncovered evidence of a possible kidnapper.

The head of the British investigation into the four-year-old's disappearance in Portugal wrote a letter to holidaymakers asking for their help, according to a newspaper.

Leicestershire Constabulary, which is co-ordinating the investigation in Britain on behalf of the Portuguese authorities, would not confirm whether Superintendent Stuart Prior had sent the appeal.

Nor would the spokeswoman confirm whether police across Britain were interviewing former guests at the Ocean Club in Praia da Luz.

The letter reportedly explains that Portuguese detectives have failed to identify a number of samples recovered from the resort. The newspaper said some of these were from the McCanns' apartment.

Superintendent Stuart Prior is said to have told tourists in the letter: "I have now been asked by the Portuguese investigation team to arrange for DNA samples and fingerprints to be taken from holiday-makers living in the UK who were staying at the Ocean Club Resort."

He goes on to emphasise that the request is "routine practice", reports say.

The development comes a month after Kate and Gerry McCann were named as suspects over the disappearance of their daughter.