The warning, delivered last week in an as-yet unpublicised judgment by Mr Justice Holman, throws doubt on another aspect of child abuse investigations - the reliance on photographs for expert opinions in cases of alleged sex abuse.

The judge said he wanted to alert doctors and lawyers to the "potential danger" in doctors' giving expert opinions based on photographs of a child's genitals taken during an initial physical examination.

In the case of a couple from the north of England with two small daughters, he said the practice had only

"narrowly avoided causing a grave miscarriage of justice and wrongly breaking up a family, perhaps for ever".

Lawyers believe this may lead to the reopening of a series of cases in which photographs were taken by colposcopy - using a probe with a digital camera attached.

The child law expert Allan Levy QC said:

"It's a cautionary tale of the highest degree. The Royal College [of Paediatrics and Child Health] and the professions generally need to consider it very carefully. Something absolutely fundamental that you've taken for granted, you've really got to look at again."

The wrong diagnosis was put right only because two of the doctors who diagnosed sexual abuse from still photographs by colposcopy decided they needed to do a physical examination when the case went to the court of appeal.

Two doctors who had initially examined the girl thought her hymen might have been torn as a result of penetration. Pictures were sent to two experts, a consultant paediatrician and a forensic physician, who also diagnosed penetration.

It was only because the case was going to the court of appeal and further reports were required that the two experts decided to examine the girl. They found that her hymen was intact and there was no evidence that she had been abused.

They said they had been misled by the pictures, and that photographs they themselves took during their examination did not match what they had seen with their own eyes.

The judge said he was not criticising any individual doctor, but added:

"The story is a very serious one. The consequences for this family have been grave and might have been even more grave." He made an order banning identification of the family.

Their ordeal began when the mother's sister told social workers that the girl, then aged two and a half, had told her "daddy hurts my bum" when she was changing her nappy. But, the mother said, the child complained to both parents about nappy-changing because she had suffered from inflammation, unrelated to abuse, from the age of nine months.

Under threat of having the children taken into care, the mother had to leave the family home and go to live with her father in another town.

The mother's solicitor, Clare Routledge, said:

"The family suffered the most appalling hardship. The local authority thought the father was a pervert. They got into debt because they were having to live apart."

The judge said that from December 2000 until the two doctors reported in June 2003, all the medical evidence was to the effect that the girl had definitely been sexually abused, with penetration.

The judge quoted the guardian appointed to safeguard the children's interests, who said:

"It is a devastating and sobering thought that had another physical examination not been carried out, the outcome to this case might have been very different."

Mr Justice Holman urged the Royal College of Paediatrics and Child Health and the Association of Police Surgeons to reconsider their joint guidance that the prime use of photos is

"to enable additional medical opinions to be obtained without subjecting the child to further examination."

He said extra examinations were "preferable to a potential grave miscarriage of justice and irreparable harm" to children and parents.

Alan Craft, the president of the royal college, said:

"We are aware of the need to review our guidance on the physical signs of child sexual abuse. As part of our review, we will be looking at how you get the evidence."