Ishbel Matheson: "As the days go by Jackie Bennett has started to fear the worst"

Jackie Bennett, 24, voluntarily put her daughters, Jade, four, and her half sister Hannah, three, in the care of Cambridge social services while she was suffering from a serious psychiatric disorder in 1997.

Detectives in Cambridgeshire believe the foster parents, Post Office worker Jeffrey Bramley, 34, his wife Jennifer, 35, must have meticulously planned their disappearance to have evaded detection for so long.

'Not fair'

In a direct appeal to the Bramleys, Ms Bennett said: "Even though I gave them up, they're still my children and I do love them and miss them.

"They are best off bringing the children back now rather than later on, because if they leave it any later it's just going to get worse.

"And it's not fair on me either, because I'm their mother."

Jade Bennett: Vanished

The Bramleys vanished with the two girls from their home in St Ives, Cambs, before they were due to meet social workers to discuss the children's future.

Mr and Mrs Bramley, who have no children of their own, may have feared that the girls were about to be taken away.

Detective Sergeant Mark Nicholson said he feared people may not be reporting sightings of the family because they sympathised with the couple's plight.

He said: "What we want to say to anyone who sees them is that it is Jade and Hannah who are suffering in this.

"The children are the victims in this and that is why we need to find this family as quickly as possible."

Police have reason to believe that the Bramleys have been to Norfolk, Kent and Sussex in the last three weeks.

But they have not ruled out the possibility that the girls have been taken abroad.

Hannah Bennett pictured with her foster mother Jennifer Bramley

Mrs Bramley's brother, Dave Bodle, 37, said: "There is no way that Jenny or Jeff would cause any harm to any child."

He said: "They obviously did want children and they were very happy to have Hannah and Jade with them."

Mr Bodle said he hoped the couple, who he described as "shy but determined", might have contacted relatives but he has heard nothing.

Mr Bramley is 5ft 6ins tall, slightly built with receding ginger hair. His wife is four inches shorter, with dark, medium-length hair and dark eyes.

They are believed to be travelling in a blue Honda Concerto, registration number G152 SHR.

But Det Sgt Nicholson said it is possible they have sold the car and taken on disguises.

He added: "It is fairly clear by now that some degree of planning had gone in to their disappearance."

Anyone who sees the couple or the children is asked to ring 01480 426111 and ask for the incident room.