It is understood a mysterious call claiming to know the whereabouts of the four-year-old came from a mobile phone registered in Argentina.

The call, described by officials as "credible", was considered so potentially significant that the McCanns halted their search of Europe to help police investigate.

The call from the pay-as-you-go phone came from a man who wanted to speak directly to the McCanns, according to Spanish police sources.

He did not reveal his identity or nationality, but the phone was soon linked to the South American country.

All efforts to re-establish contact with the caller failed on Wednesday and the couple carried on with their journey around Europe.

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

Spanish officials at first denied they had received the call, but a Guardia Civil source told the Portuguese paper Correio de Manha: "Only time will tell if this call gives help or not to the case."

Spanish newspaper El Mundo reported that a man matching the description released by Portuguese police two weeks ago was seen in a bar in Seville a week before Madeleine's abduction.

It claimed the man was working on the instruction of others and told fellow drinkers he was going to the Algarve.

In Praia da Luz, Mr and Mrs McCann planned a trip to Morocco as the final part of their attempt to raise awareness of their daughter's disappearance.

They are keen to go because of its proximity to Portugal. Several ferries a day leave from Tarifa in Spain for the 35-minute crossing to Tangiers.

The couple are considering catching the ferry and travelling to the capital Rabat.

Morocco has surfaced repeatedly in the hunt for Madeleine, with a reported sighting of a little girl seen with a man.

It is 35 days since Madeleine was abducted from her bed in the Portuguese resort of Praia da Luz.

Police have only one formal suspect, Robert Murat