Camden Council said it would give the parents one more chance to have their daughter tested before taking the case back to court. The authority urged the parents last night to consider their position very carefully.
There are fears that the five-month-old child has been taken abroad to avoid complying with the High Court order.
Camden Council, which successfully applied to the court for an order that the child should be tested under the 1989 Children Act, admitted it had no idea where she was.
Three weeks ago the High Court agreed that the parents, who cannot be named for legal reasons, did not have the ultimate right to deny their child possibly life-saving medical treatment.
The girl's parents, from north London, are believed to have gone into hiding just days before they were due to start their appeal against the court's decision.
The Court of Appeal backed the High Court decision on Monday and the parents were due to take the girl to Great Ormond Street Hospital in central London yesterday afternoon, but failed to arrive.
The council took the case to court after failing to persuade her HIV positive mother and the father, who is clear of the virus, to have her tested.
At the time of the court case, the mother said: "We don't want state intervention in our daughter's upbringing. If you take an action that upsets parents it will upset the child."
But the council argued that children born with HIV usually develop Aids. If the baby is found to be HIV positive, it wants her to be treated with drugs which slow the onset of the disease.Reuse content