They did so while disregarding warnings from independent advisers that the much-debated treatment hasn’t been shown to help slow the brain-destroying disease.

The Food and Drug Administration said it granted approval to the drug from Biogen based on results that seemed “reasonably likely” to benefit Alzheimer’s patients.

It’s the only drug that US regulators have said can likely treat the underlying disease, rather than manage symptoms like anxiety and insomnia.

Aducanumab targets amyloid, a protein that forms abnormal clumps in the brains of people with Alzheimer’s that can damage cells and trigger dementia.

The decision to approve it, which could impact millions of older Americans and their families, is certain to spark disagreements among physicians, medical researchers and patient groups.

It also has far-reaching implications for the standards used to evaluate experimental therapies, including those that show only incremental benefits.

The new drug, which Biogen developed with Japan’s Eisai Co, did not reverse mental decline, only slowing it in one study.

The treatment could lead other pharmaceutical companies to create similar drugs and the UK regulator, the Medicines and Healthcare products Regulatory Agency (MHRA), to consider it for use in Britain.

At least 100,000 people in the UK with a mild form of the disease could be suitable for the drug if it were to be approved by the UK regulator, the BBC reported.

Alzheimer’s Research UK, said it had written to the Health Secretary Matt Hancock asking the government to prioritise the fast-track approval process for the drug in the UK.

Chief executive Hilary Evans, said: “People with dementia and their families have been waiting far too long for life-changing new treatments.

“It is now essential that regulatory authorities assess the evidence to decide whether they believe the drug is safe and effective for use in the UK.”

Alzheimer’s Society said the drug was "promising" but added it was "just the beginning of the road to new treatments for Alzheimer’s disease".

The medication, aducanumab, will be marketed as Aduhelm and is to be given as an infusion every four weeks.

Dr Caleb Alexander, an FDA adviser who recommended against the drug’s approval, said he was “surprised and disappointed” by the decision to approve it.

“The FDA gets the respect that it does because it has regulatory standards that are based on firm evidence. In this case, I think they gave the product a pass,” said Dr Alexander, a researcher at Johns Hopkins University.

The FDA’s top drug regulator acknowledged in a statement that “residual uncertainties” surround the drug, but said Aduhelm’s ability to reduce harmful clumps of plaque in the brain “is expected” to help slow dementia.

Under terms of the so-called accelerated approval, the FDA is requiring the drugmaker to conduct a follow-up study to confirm benefits for patients.

If the study fails to show effectiveness, the FDA could pull the drug from the market, though the agency rarely does so.

Nearly six million people in the US and many more worldwide have Alzheimer’s, which gradually attacks areas of the brain needed for memory, reasoning, communication and basic daily tasks.

In the final stages of the disease, those afflicted lose the ability to swallow.

The global burden of the disease, the most common cause of dementia, is only expected to grow as millions more baby boomers progress further into their 60s and 70s.