The mystery call that police believe may contain “credible” information on the disappearance of Madeleine McCann has been traced to a mobile telephone registered in Argentina.

Detectives hope that the caller may hold the clue to the whereabouts of the four-year-old, who was abducted from the bed of her Algarve holiday apartment 37 days ago.

Kate and Gerry McCann still hope contact can be re-established before tomorrow when they travel to Morocco in the final of a series of visits to raise awareness of their daughter’s disappearance. They will then remain in Portugal, co-ordinating the search for Madeleine.

Although aware of the possibility that their daughter could be dead, they believe it is more likely that they are just one telephone call from having her returned home safely. The couple hoped that such a message had been received on Wednesday when British police told them that a “credible” call had been received by the Guardia Civil in Spain from someone claiming to have information about Madeleine’s whereabouts.

Detectives are still investigating the call, which they have discovered was made on a telephone registered in Argentina. They have emphasised that it could have been made elsewhere in the world.

A police source said: “The call came from an unregistered [pay-as-you-go] phone but there was sufficient information to know where it was bought or registered.” Authorities in Buenos Aires are understood to be attempting to trace the owner of a pay-as-you-go mobile registered with an operator in Argentina.

The caller is believed to have demanded to speak to the McCanns and a time was arranged for them to accept the call. The call was considered so potentially significant that the McCanns delayed a trip to Amsterdam, to help police investigate.

A British police source said: “The importance of this line of inquiry is still being assessed and attempts to re-establish contact are continuing.”

Although Spanish officials denied the existence of a call, a Guardia Civil source confirmed that it had been received. He told the Portuguese paper Correio da Manhã: “Only time will tell if this call gives help or not to the case.” The newspaper said that the caller appeared more credible than the hundreds of others because of the “amount of detail given”.

The McCanns, from Leicestershire, have decided to stay in Praia da Luz throughout the summer in the hope that Madeleine will be found. A family source said: “They don’t put a timescale on it but are prepared to live here until the end of the summer at the earliest. They are getting semi-regular meetings with Portuguese police and want to maintain that contact, and they are happy with the co-operation back home. The source said that even if Madeleine was “somewhere else physically, this is the place where mentally they make a connection. They can’t go from here”.

Mr McCann, 39, wrote in his internet blog findmadeleine.com yesterday: “After Morocco we will have to take stock and decide on our best strategy.”

Mrs McCann, 38, is concerned at the lack of publicity in North Africa despite the ease of access from Portugal.

The couple believe that Madeleine could have been taken out of Portugal soon after her disappearance and are alarmed at the refusal of the main airport, in Lisbon, to publicise the search. Baroness Ashton of Upholland, a justice minister, has said she will raise the issue with her Portuguese counterpart.

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