Gerry and Kate McCann said five-year-olds Sean and Amelie were old enough now to know something was wrong in the family, and Mrs McCann said they were asking questions that needed to be answered.

She told GMTV's Lorraine Kelly: 'I think it was last year Amelie asked me, "Has Madeleine run away?"... and she said "because it's not nice to run away".

Disappointed: on the eve of the third anniversary of Madeleine's disappearance, Gerry and Kate McCann say police should be doing more to solve the case

'That really upset me because I thought I don't want her to think that Madeleine is at fault. So, probably about the third time she asked when we were at home, we just explained that someone had taken Madeleine.

'But we tried to make them understand in as gentle a way as possible. It's a bit like stealing, you know? That's how the understand it.

'So they know someone has taken her and they know it is wrong.'

Mrs McCann's heartbreaking comments come on the day the couple accused British police of 'giving up' on their missing daughter.

The McCanns, both 41, spoke out just days before the third anniversary of Maddie's disappearance, urging investigators to go back to the start and review the case.

They fear their own ?2million search - funded by public donations received since Maddie vanished from their Algarve holiday apartment - has stalled.

Madeleince McCann: The couple spoke out just days before the third anniversary of her disappearance

They claim this is due to the failure of both Portuguese and UK police to investigate leads unearthed by their private investigators.

In a pre-recorded GMTV interview due to be broadcast today, Mr McCann said: 'It's not right that an innocent, vulnerable British citizen is essentially given up on. And I don't think it's right that, as parents, that we have to drive the search.'

He added: 'We need to have a proper review of all the information - that's how we will move the investigation forward.'

Mrs McCann, a GP who gave up work to concentrate on the search for Madeleine, said: 'We do this in medicine. You know, if there is a case that you don't seem to be getting the diagnosis, somebody will come in and review it. They'll go back to square one... and that's where you find out what else needs to be done and it will help point you in the right direction.'

Leicestershire Police have carried out their own inquiries, as has the taxpayer-funded Child Exploitation and On-Line Protection Centre. But neither is actively seeking the little girl.

The three-year saga has already cost UK taxpayers nearly ?500,000.

The couple, from Rothley, Leicestershire, said it was 'incredibly frustrating' that police in Portugal and the UK are not doing more to find Maddie, who was three when she disappeared from the holiday complex at Praia da Luz on the evening of May 3, 2007.

The McCanns were criticised for leaving the girl and their twins Sean and Amelie, then two, alone in the apartment while they dined with friends at a tapas restaurant 40 yards away.

In the latest interview, Mr McCann, a consultant cardiologist, said that if they could go back, they would not leave Maddie alone.