Tens of thousands of people will be recruited into Alzheimer's trials after a landmark drug was rejected for use on the NHS.
Scientists have promised a mass expansion in research as annual figures showed just 61 patients in the UK took part in trials.
The National Institute for Health and Care Excellence (Nice) confirmed on Wednesday that it would not fund donanemab - the most effective drug yet to slow progress of Alzheimer's disease - as it was licensed by safety regulators.
In the US, the drug is estimated to cost around £25,000 per patient annually, but treatment costs on top of that include monitoring and scans, amounting to around £60,000 per patient.
Nice said the costs could not justify the benefits, estimating that the monthly injections slowed cognitive decline by around four to seven months.
Charities said the announcement was "another frustrating setback" for people affected by Alzheimer's disease, describing the decision as "incredibly disappointing".
Hilary Evans-Newton, the chief executive at Alzheimer's Research UK, urged Wes Streeting to "break the deadlock we're facing, where research is delivering new treatments but they remain out of reach for NHS patients".
The charity highlighted the fact that NHS England has identified nearly 30 more dementia treatments that could be available by 2030, calling for leadership to bring Nice, NHS England and industry together "so that people with dementia in the UK aren't left behind".
Lord Cameron, the former prime minister, said the decision by Nice was "disappointing and disheartening".
"We urgently need to find a way to diagnose patients more quickly and make this and other future medicines readily available to all," he said.
The decision mirrors one taken in August over lecanemab, a drug which similarly works to clear the brain of plaque.
It means that the only way people with Alzheimer's disease will be able to obtain such treatments, and others in the pipeline, is through private clinics, or as part of clinical trials.
The UK Dementia Research Institute announced a £20 million initiative on Thursday to boost the numbers receiving medicines through trials.
The Dementia Trials Accelerator, funded by the Medical Research Council, will allow "tens of thousands" to take part, scientists say.
Figures suggest that between 2021-22, only 61 people took part in dementia trials.
The Institute said one of the key challenges to breakthroughs in dementia treatments is getting large enough numbers of volunteers to come forward, and ensure enough get through screening procedures.
Nice has been criticised because its decisions do not take account of the £42 billion wider costs of dementia to the economy, and the unpaid burden of labour on families.
Campaigners said the Government was willing to spend on weight loss jabs to get younger people back to work, in order to boost the economy, while paying little heed to the difference that breakthrough Alzheimer's drugs could make, for older generations.
Last week Mr Streeting suggested that the injections could have a "monumental" impact on obesity, and get more people back to work.
Dennis Reed, from Silver Voices, a campaign group for the over 60s, said: "We've got a million people who are living with dementia at the moment, and an awful lot of these could have a longer and more fulfilling life, thanks to this suite of drugs."
"The Government thinks about the economy when it comes to younger people, and ideas like weight loss jabs to get them back to work. But they aren't prepared to look at that social and economic side when it comes to older people. A lot of our members feel they are just being put on the scrap heap: that once they stop working, that they're basically not worth investing in anymore and become second class citizens."
The drug was licensed for patients who have one or no copies of the apolipoprotein E4 gene on Wednesday.
It is estimated that around 70,000 adults in England would have been eligible for treatment with donanemab.
But Nice said the drug did not "currently demonstrate value for the NHS".
Its independent committee found the medicine could slow down cognitive decline by four to seven months, but said this was not enough to justify the spending.
Helen Knight, the director of medicines evaluation at Nice said: "For Nice to be able to approve a medicine for use in the NHS it must provide additional benefits to patients, and it must also represent a good use of NHS resources and taxpayers' money.
"Our independent committee looked at all the available evidence, including the benefits for carers. This shows donanemab could slow down cognitive decline by 4-7 months, but this is just not enough benefit to justify the additional cost to the NHS. The cost-effectiveness estimate for donanemab is 5 to 6 times above what Nice normally considers an acceptable use of NHS resources.
"I know this will be disappointing news, but this is an emerging field of medicine and there are other treatments being developed.
A Department of Health and Social Care spokesman said: "These are very difficult decisions to make and it is right that they are taken independently, based on an assessment of the available evidence on the relative costs and benefits of a treatment.
"The Government is committed to continuing to expand research and innovation in this area, and will continue to work with NHSE and Nice to make treatments available as and when they are assessed to be ready."