The European Medicines Agency has said no to approving a new drug for Alzheimer's disease in the EU.
Aducanumab does not appear to be effective at treating adults with early-stage symptoms, the EMA said.
The drug - the first new treatment for 20 years - was controversially approved in the US in June.
At the time, many scientists said there was little evidence from trials of it being of benefit despite it targeting amyloid - a protein that forms abnormal clumps in the brains of people with Alzheimer's.
Biogen, the manufacturer of the drug, can ask for the EMA's decision to be re-examined within the next two weeks.
It's not known whether the company will submit a separate application for approval to the UK regulator, the MHRA.
'Could cause harm'
The European Medicines Agency based its decision on two main studies of more than 3,000 patients with early-stage Alzheimer's disease which compared the effects of a low dose and high dose of the drug with a dummy pill.
"Results from the main studies were conflicting and did not show overall that aducanumab was effective at treating adults with early-stage Alzheimer's disease," the EMA said.
"In addition, the studies did not show that the medicine was sufficiently safe as images from brain scans of some patients showed abnormalities suggestive of swelling or bleeding, which could potentially cause harm," it added.
The EMA concluded that the benefits of the drug did not outweigh its risks and recommended the refusal of a marketing authorisation.
There are estimated to be nearly 8 million people living with dementia in the EU and around a million in the UK, with numbers expected to double by 2050.
Alzheimer's disease is thought to be the cause of the majority of those cases.