McCanns admit that they have thought the unthinkable about missing Madeleine

Asked about the possibility that the four-year-old may be the prisoner of a paedophile, Kate McCann struggled to control her emotions before replying: "It is a more upsetting scenario that any other.

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"We have considered all scenarios, as everyone has."

She and her husband Jerry cling to the hope that Madeleine, who disappeared from their holiday apartment in Portugal on May 3, is being looked after well by someone who desperately wanted a daughter.

Mr McCann said: "We still very much focus on the positives. We pray that the person who has her gives her up voluntarily and drops her at a church or a safe place."

As the couple spoke, detectives in Portugal were said to be trying to identify the DNA of a stranger found in the McCanns' holiday apartment at Praia da Luz. It did not match that of chief suspect Robert Murat.

In Madrid, the McCanns were handed a letter from the family of Yeremi Vargas, a ten-year-old boy who disappeared on Gran Canaria in March.

The gesture reduced Kate McCann almost to tears as it became poignantly clear that the two mothers have been sharing their agony in parallel.

The letter said: "There is no one better than us to know what you are living through. Don't you ever lose hope, which is what is left to us.

"While there is no proof about how Madeleine is, we can only believe she is just missing.

"Count on us for anything you may need. Be strong and keep the family together. It is the best thing that we have. May God walk along with you in this hard road. Many kisses, and warmest regards, from Yeremi's family."

The McCanns, holding hands throughout, spoke at length to the Spanish TV programme Los Mas Buscados, or Most Wanted, a daily hour-long show about missing people.

Mr McCann said child disappearance cases throughout Europe should be studied together.

The couple, both 38-year-old doctors from Leicestershire, also met campaigners against child pornography before returning to Praia da Luz and their two-year-old twins Sean and Amelie.

Next week they plan to go to Berlin, Amsterdam and Morocco.