The NHS watchdog has been accused of being "nonsensical" for denying young children with severe asthma access to a drug.
The National Institute for Health and Clinical Excellence (Nice) has refused Xolair (omalizumab) for use on the NHS for children aged six to 11.
It says its high cost does not justify its "little extra benefit compared with existing treatments".
The medicine could be used for up to 300 children in the UK.
The drug is injected into the skin to control severe allergic asthma which has not responded to conventional treatments. It is already approved for adults and children over the age of 12.
Nice has rejected it for younger children in draft guidance which is subject to appeal.
Charity Asthma UK condemned the decision, saying children on the drug in UK trials "have experienced massive improvements to their quality of life".
Dr Mike Thomas, chief medical adviser to Asthma UK, said: "Hundreds of children across England with the most severe, allergic asthma will now be denied a pioneering treatment that could free them from crippling daily asthma symptoms, endless trips to hospital and huge amounts of time off school."
The drug has been approved for young children in Scotland, but that is likely to be reviewed following the decision.
Dr Thomas said it was vital the children who were being treated with the drug did not have it withdrawn, as it would be "completely unjust".
John O Warner, professor of Paediatrics at Imperial College London, said Nice had based its decision on a study submitted by the manufacturer, Novartis, which had a too-small sample.
"Given that the asthma quality of life improvements were equal to those in adults and the demonstration of highly beneficial and clinically meaningful improvements in children with severe asthma aged 6 to 11, it is a great tragedy that Nice have failed to approve Xolair.
"It is nonsensical that Xolair will be available to children aged 6 to 11 in Scotland but not in England. It is also nonsensical that it can be administered on the NHS to a child of 12 but not to one of 11."
Dr Gillian Leng, Nice deputy chief executive, said it had considered all the research.
"We are unable to recommend that NHS funds be diverted to a treatment with such high costs which only provides very limited benefits for patients."
She said the recommendation was not based on the age of patients, but rather on the fact the drug had not been shown to be cost-effective for those under 12.
Xolair costs £256.15 plus VAT per vial.