There are officially about 1,500 children in private foster care in the UK
Thousands of children in the UK may be being unlawfully fostered and could be at risk of abuse, a charity has warned.
Under the law, the authorities have to be informed if a child is looked after by anyone other than a close relative.
The British Association for Adoption and Fostering (BAAF) said this was not happening and children in private foster care were "invisible".
The charity is urging people to report neighbours who are caring for children unrelated to them on a long-term basis.
Private fostering is when a child is looked after by adults who are not close relatives - defined as grandparents, aunts, uncles and siblings - and it can be a useful and less formal alternative to foster care.
While most children in private fostering situations will be well cared for, some may not be. It is those children we are concerned about
David Holmes, BAAF chief executive
Officially, there are about 1,500 children currently in private foster care in the UK but the authorities believe the actual number to be much higher.
In 2004, the Department of Health estimated it to be nearer 10,000 in England and Wales, while the BAAF said it was very difficult to be accurate.
They include children whose parents are in prison, runaway teenagers, children sent to the UK for education or who have been brought here by traffickers.
A child can live away from home as long as parental approval is given, but after 28 days the new carer must alert social services.
The BAAF has launched a national awareness campaign to get the message across that in private fostering situations, the local authority must be informed.
The worst moment was when my brother and I were forced to drink our urine because we had wet our bed
David Holmes, chief executive of BAAF, said: "While most children in private fostering situations will be well cared for, some may not be. It is those children we are concerned about.
"Therefore if people suspect a child is being privately fostered in their local community, please do not to ignore it. Either talk to your neighbour or, if appropriate, call your local council."
Former Olympic athlete Kriss Akabusi told the BBC of the physical abuse he and his brother suffered when they were privately fostered as children in the 1960s.
Their parents left them in the care of foster-families, while they returned to "nation-build" in Nigeria after independence.
Mr Akabusi said: "One woman was very brutal and was battering us all the time.
"I remember seeing my brother getting battered and being the older brother I was supposed to protect him, but I couldn't do anything about that.
"The worst moment was when my brother and I were forced to drink our urine because we had wet our bed."
He said he had come through the experience relatively unscathed, but that the repercussions in later life had been "more serious" for his brother.
"I have a good outlook because I survived the experience," he said.
Olympic medallist Kris Akabusi was abused when fostered privately
"But not all kids do survive and many fall through the cracks."
Research carried out on the BAAF's behalf revealed 22% of adults would do nothing if an unrelated child suddenly moved in next door, and 7% would not act if they disappeared.
The survey suggested the most common reason was people felt it was none of their business.
The research was carried out by ICM who polled 1,979 adults online between 5 and 7 December last year.
Measures to regulate private fostering were brought in after the death of Victoria Climbie, the eight-year-old who was murdered by her great-aunt Marie Therese Kouao and her boyfriend Carl Manning in 2000.
She had been sent to the UK from the Ivory Coast by her parents, who thought she would have a better life.
The government says an expert advisory group is currently looking at how to make sure all private fostering arrangements are made public.
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