Vacationers who stayed at the resort where Madeleine McCann disappeared are being asked to provide DNA samples and fingerprints after detectives found evidence of a possible kidnapper, The Times can reveal.

A letter from the head of the British investigation says that Portuguese detectives have been unable to identify a number of samples recovered at the Ocean Club in Praia da Luz.

Police across Britain are now interviewing former guests in the biggest operation since the search for Madeleine was abandoned three weeks after her disappearance in May.

The move came as Paulo Rebelo, one of Portugal’s most senior detectives, was put in charge of the case and as officers were reported to have undertaken a new search of the McCanns’ two-bedroom apartment and the beach near by.

The Times has learnt that detectives recovered unidentified DNA and fingerprints during a series of earlier searches. They have also been unable to trace a number of mobile telephone calls made on the evening that Madeleine was reported missing, six days before her fourth birthday.

The operation — which comes a month after Kate and Gerry McCann were named as suspects over the disappearance of their daughter — suggests that detectives cannot rule out the possibility that a stranger might have stolen the child from her bed.

Leicestershire Constabulary, which is co-ordinating the investigation in Britain, was officially asked by the Portuguese authorities last month to help. In a letter obtained by The Times, Detective Superintendent Stuart Prior tells the tourists: “I have now been asked by the Portuguese investigation team to arrange for DNA samples and fingerprints to be taken from holidaymakers living in the UK who were staying at the Ocean Club Resort.”

He wrote that the DNA and fingerprints “will help the Portuguese police to eliminate samples they have taken and not yet been able to identify”. The tourists are also being asked about the use of their mobile phones and for details of their movements and clothing the evening that Madeleine disappeared.The DNA samples will be analysed by the Forensic Science Service in Birmingham, which has carried out tests on material recovered from Ocean Club flats and several vehicles, including the McCanns’ hire car.

Mr Prior’s letter says that the operation was routine practice and that the DNA samples would not be added to any national database in Britain.

One of the tourists already interviewed by police said that British officers had criticised the decision to wait so long to mount such an operation. “They said that they would aim to do this kind of work within 48 hours of a disappearance and could not comprehend why it has taken five months. We struggled to remember the kind of detail that they wanted about our movements on specific days and what we were wearing.”

The questioning of the British tourists was requested last month by Luis Bilro Verão, the prosecutor overseeing the case, after a judge ruled that there was insufficient evidence for him to order the McCanns to return to the Algarve for further questioning.

Clarence Mitchell, the McCanns’ spokesman, said he hoped that Mr Rebelo’s first priority as head of the investigation would be to eliminate the couple from the inquiry and refocus efforts on finding Madeleine.