Charlotte Wyatt - the baby at the centre of a right-to-life legal row - remains in a "serious but stable" condition after her health worsened.
On Thursday, a High Court judge ruled that doctors could let the two-year-old die after it was decided she was on a "downward rather than an upward trend".
The ruling follows a series of legal battles over the treatment of the severely disabled Portsmouth toddler.
Charlotte now has an "aggressive viral condition" clogging her lungs.
Mr Justice Hedley heard an emergency application from St Mary's Hospital in Portsmouth on Thursday night.
He also heard that Charlotte's mother, Debbie, believed the child could recover if doctors helped her breathe by inserting a tube in her windpipe.
But in his judgment he said: "Medical evidence speaks with one voice, that ventilation simply will not achieve the end for which no doubt the parents would wish and indeed that Charlotte would be unlikely to survive such a procedure."
Charlotte was born three months prematurely with serious brain, lung and kidney damage.
Her parents have fought a series of court battles with Portsmouth NHS Trust over whether Charlotte should be given artificial ventilation if her condition worsened.
In October last year, her parents won a partial victory when a judge lifted an order saying doctors would not be acting unlawfully if they decided not to give Charlotte artificial ventilation in a life-threatening situation.
The judge said then that her parents should reach agreement with the doctors about their daughter's treatment if a crisis arose.
Charlotte's condition had improved significantly in recent months and staff at the hospital, where she has remained since she was born, even said she may be able to go home permanently.
She was allowed home for the first time unsupervised on Christmas Day, 2005.
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