The parents of Madeleine McCann have given an insight into the family's nightmare since their daughter was abducted three weeks ago.

Gerry and Kate McCann have told Sky News about their guilt at leaving Madeleine and her twin brother and sister alone in their bedroom, the police investigation and the support they've received from people around the world.

Kate told Sky's Ian Woods of the moment three weeks ago when she found Madeleine had vanished.

"We were checking regularly on the children and it was during one of my checks that I discovered she was gone. I can't really go into any details about that. I'm sure any parent will realize how that felt."

The panic set in "pretty much immediately," she said.

They explained why their three children were left alone while they had a meal nearby.

"I think If you know the location here - what we did we've been assured by the thousands of messages from people who've either done exactly the same or would have done the same. For us it really wasn't very much different from having dinner in your garden, in the proximity of the location," Gerry said.

"I think it's fair to say that the guilt that we feel, having not been there at that moment - irrespective of whether we had been in the other bedroom or not - will never leave us."

Kate said they blamed themselves for the disappearance in the early stages.

"Certainly in the first few days the guilt was very difficult. I think as time goes on we feel stronger and we've felt very supported. At worst we were naive. We are very responsible parents. We love our children very much. And I don't think that any parent could ever imagine or consider anything like this happening," Kate said.

The couple said they had been aware of the big public debate which followed the abduction and admitted they were hurt by it.

"No one hurts you as much as the hurt we had. We've tried to remain very positive in our outlook, and even small levels of criticism make that hard when you are trying to do everything in your power to get your daughter back," Gerry said.

Of the police investigation, Gerry said: "I don't think it helps to look back at what could and couldn't have been done. I think it's fair to say that we expected a very British-style response that you would expect if you were in a big metropolitan city.

"But you have to put that in context - we're in a tiny resort. But that aside, the time for the lessons to be learned will be after the investigation has finished and not now.

Gerry said it was a huge investigation and there are hundreds of pieces of information continuing to come forward.

He asked that any vacationers who were in the Algarve in the two weeks before Madeleine disappeared to send in their photos, using the website www.madeleine.ceopupload.com

"We want Madeleine back and people can still influence that," said Gerry, a hospital consultant.

The McCanns said they had not seen anything suspicious in the days before Madeleine's disappearance.

"It had been such a relaxing holiday up to that point - as good a holiday as we ever had," said Gerry.

The couple said they still believe Madeleine will be found safe but admitted to having negative thoughts in the early days.

Kate said: "We're actually a lot stronger, a lot more hopeful now. And we have to be hopeful - it's what keeps us focussed."

Kate said the twins, two-year-olds Sean and Amelie, were of an age where the disappearance had not had such an impact on them.

"They do talk about Madeleine - pick up things and say "Madeleine's." They're really good."

Gerry said: "Many people have said to us this is a parent's worst nightmare, and it is - it truly is and it's as bad as you could possibly imagine. But if all three of the children had been taken it could have been even worse than your worst nightmare.

"And we've got to be strong for them. They are here. They bring you back to earth."

Kate said she couldn't think about going home without Madeleine.

She spoke of the comfort she gets from Madeleine's 'Cuddle Cat' toy she carries.

Gerry explained why he thought the story had attracted so much worldwide attention.

"The world has changed in terms of information technology and the speed of response, you know in terms of the media coming here and us being prepared to some extent use that to try and influence the campaign.

"But above all else it's touched everyone. Everyone. You don't have to be a parent for this to have a major impact on you."

The couple recognized that the level of media coverage will not last forever but said they had sensed a feeling that people would not allow something like this to happen again.