A mother who faces her baby being taken away for adoption has failed in a last-ditch bid to prove the child suffered two fractures because of an extremely rare bone disease.

A top judge in London's Court of Appeal said the circumstances surrounding the 11-month-old boy's injuries remained a "deep mystery", noting there were four potential perpetrators - the parents and the two grandmothers.

Lord Justice Thorpe refused the mother's plea to have genetic testing carried out on her son, which would cost £5,000 and take six weeks, to determine whether he suffers from osteogenesis imperfecta, which causes bones to break very easily.

The judge noted an expert on the disease had already concluded such a diagnosis was "extremely unlikely"

given there was no family history or clinical signs of the disease.

The judge said: "There has to be a point at which the garnering of evidence is sufficiently full and thorough for the courts to arrive at a conclusion."

The baby, who cannot be named for legal reasons, was taken to the Royal Sussex County Hospital's accident and emergency department last year after his mother noticed he was more tearful than normal.

Lord Justice Thorpe said X-rays showed he had a "rare" fracture of the ulna and a "commonplace" tibial fracture, although the reasons for the broken bones remained a "deep mystery".

The judge said: "As so often is the case, the parents are distraught and have no seeming explanation.

They can't recall a demonstration of the extraordinary pain that the child inevitably experienced."

Brighton and Hove City Council intervened to take the baby from his parents and place him in temporary foster care.

At a preliminary hearing in Brighton County Court, barrister Francesca Wiley, for the mother, introduced the possibility of osteogenesis imperfecta. The judge allowed an expert to assess the baby but, after reading his report, refused the genetic test and the mother took her fight to the Court of Appeal.

In rejecting her appeal Lord Justice Thorpe pointed out the "extensive blood test" was only accurate in 90 per cent of cases. He said the benefit was too small to justify not only the cash and time spent on the test, but also to subject the baby to such an invasive investigation.

The hearing will continue at county court later this year and, if the mother is unsuccessful, the baby is likely to be taken from her permanently and put up for adoption.