The number of people suspected of having dementia who are undergoing assessments in order to get a diagnosis has halved since the pandemic, official figures show.
Charities warned the “alarming” plunge in numbers means tens of thousands of people with conditions such as Alzheimer’s disease are not being offered support or medication.
It follows warnings that patients are waiting more than a year for help.
When GPs suspect dementia, they are supposed to refer patients to a memory clinic, which can make a diagnosis.
In 2019, there were almost 56,000 such assessments in England.
But since the pandemic, that figure has halved. If services had continued to function normally, around 140,000 such assessments would have taken place since the start of 2020. But data published by the NHS shows fewer than 73,000 consultations have taken place.
Experts said some of the decline could be explained by elderly people failing to seek help during the pandemic, with some fearing they would be a burden on their GP.
But separate figures show that even when help was sought, many people with suspected dementia have still faced long waits to be assessed by a clinic.
And data for the six months ending in June show that even now, clinics are only operating at 58 per cent capacity, compared with the same period before the pandemic.
PM launches 'Babs' Army' in memory of Barbara Windsor
It comes as Boris Johnson is launching a new "national mission" to tackle dementia in memory of the late Carry On and EastEnders star, Dame Barbara Windsor.
The Prime Minister said the Government would commit an additional £95 million in research funding, meeting a manifesto commitment to double funding into seeking treatments for the disease to £160 million by 2024.
He issued an appeal for a “Babs’ Army” of volunteers, with or without a family history of dementia, to step forward to take part in clinical trials on new preventative therapies.
The announcement follows a meeting earlier this week in Downing Street between Mr Johnson and Dame Barbara's widower, Scott Mitchell.
The actress, who died in 2020, helped spearhead a campaign to raise awareness of dementia after Mr Mitchell disclosed in 2018 that she had been diagnosed with the disease four years earlier.
Mr Johnson said: “Dame Barbara Windsor was a British hero. I am delighted that we can now honour Dame Barbara in such a fitting way, launching a new national dementia mission in her name.”
An investigation by The Telegraph earlier this year found there were 13,800 patients on waiting lists for memory clinics, which diagnose conditions such as Alzheimer’s disease.
Figures from 28 NHS organisations – a fraction of the 200 providing such services – showed more than 2,500 patients were facing waits of at least six months. At the worst-performing trusts, the average wait was a year.
Trusts with the longest waiting times blamed short-staffing, with some saying their backlogs had grown while staff were redeployed to help with other services during the pandemic.
The NHS does not publish waiting times for dementia patients, which charities have criticised, saying it is symptomatic of a deeper neglect of the most vulnerable.
James White, the head of public affairs and campaigns at Alzheimer’s Society, said: “The pandemic is still causing major disruption and this drop in assessments is particularly alarming.
“There has been considerable improvement in waiting times for elective surgery thanks to targeted efforts, while memory assessments for dementia are only operating at 58 per cent capacity compared to pre-pandemic levels. We cannot afford to treat dementia, which is the biggest killer in the UK, as an afterthought.
“Behind this data are tens of thousands of people feeling anxious, confused by symptoms and desperately worried about the future. We know how vital getting that diagnosis is – nine in 10 people told us they felt the benefit – unlocking the door to treatment, care and support.
Falling rates of diagnosis since the first lockdown amount to 33,000 fewer cases spotted, the charity said, calling for an extra £70 million investment to tackle the problem.
Mr White said: “The pandemic has had an unprecedented impact on dementia diagnosis. Diagnosis rates have plunged to a five-year low and we estimate at least 30,000 extra people need to be diagnosed to get us back to pre-pandemic levels.
“This means tens of thousands of people across the UK are living each day with undiagnosed dementia, shut out from the benefits of vital treatment and support.”
Health officials said the total number of referrals to clinics for all types of memory conditions was higher in the past three months compared with the same period in 2019.
Prof Alistair Burns, the NHS national clinical director for dementia, said: “There are many things we can do in the NHS to care for and support people if they do get a diagnosis, and importantly, there is support for their families and carers too.
“So if you or someone you know is worried about these symptoms, please approach your GP for an assessment. The sooner we see you, the sooner we can support you and your loved ones.”