The mother of 12-year-old Archie Battersbee has said she is "devastated" after a judge at the Royal Courts of Justice ruled that his life support should be removed.
Permission was sought from the courts by doctors caring for Archie at the Royal London Hospital.
Mrs Justice Arbuthnot ruled that "on the balance of probabilities", Archie had died and that it would be "lawful" to withdraw his life support.
Speaking after the verdict, Archie's mother, Hollie Dance, said she felt "devastated and extremely disappointed".
"Basing this judgment on an MRI test and that he is 'likely' to be dead, is not good enough. This is believed to be the first time that someone has been declared 'likely' to be dead based on an MRI test," she said.
"The medical expert opinion presented in Court was clear in that the whole concept of 'brain death' is now discredited, and in any event, Archie cannot be reliably diagnosed as brain-dead.
"I feel sickened that the hospital and the Judge have failed to take the wishes of the family into consideration."
The family plans to appeal the ruling.
"I do not believe Archie has been given enough time. From the beginning I have always thought 'why the rush?' His heart is still beating, he has gripped my hand, and as his mother, I know he is still in there," said Ms Dance.
"Until it's God's way I won't accept he should go. I know of miracles when people have come back from being brain dead.
"This case raises the significant moral, legal and medical questions as to when a person is dead. What does this ruling today tell us about where our society is at?
"We intend to appeal and will not give up on Archie."
In submissions heard at the High Court last week, Bruno Quintavalle, acting on behalf of Archie's parents, said that it should be beyond reasonable doubt that Archie is dead.
"That the court should declare, in the absence of any certainty, that death has occurred is an extremely serious issue," he said.
"If he is declared dead but actually isn't dead, the consequences couldn't be more grave.
"If someone is alive when organs are harvested (for donation), then that act of removing a beating heart will kill them."
The family are being supported by the Christian Legal Centre.
Andrea Williams, CLC chief executive, said, "This ruling is a devastating moment for Archie and his family.
"The idea that death can be declared on the balance of probability is frankly ludicrous. Life is the most precious gift that we have. This ruling sets a troubling and dark precedent.
"This case has raised significant moral, legal and medical questions as to when a person is dead. Archie's parents do not accept that he is dead and are fighting courageously for his life.
"They will not give up now and intend to appeal. We will continue to stand with the family and continue to pray for a miracle."