The Age UK review identified more than three million people aged over 65 with a care need, but found just two-thirds of them were actually getting help.
The charity warned that the lack of support for the ageing population was risking their health.
But ministers said steps were being taken to provide more help.
The review used official data and existing research to identify how many people were struggling with everyday tasks and how many were getting help.
There are 10 million people over the age of 65 in England, the review said, and more than 3 million struggle with tasks such as washing, dressing, eating and going to the toilet.
Just over one million pay for care or rely on family and friends with another 850,000 supported by their local councils.
But that leaves another one million who have to fend for themselves.
The report also warned that community NHS services and GPs were struggling to meet demand from the ageing population.
And Age UK warned there were signs this was affecting the health of older people.
It cited an 88% rise in hospital admissions for urinary tract infections among the over-75s between 2005-06 and 2013-14 to 4,173 per 100,000.
Meanwhile, admissions for pneumonia among the over-60s have more than doubled over the same period to 2,621 per 100,000.
Age UK charity director Caroline Abrahams said it was a "destructive vicious circle" where the lack of support was worsening the health of the most vulnerable older people in society.
"If an older person asked us today how confident we were that their health and care needs will be met well in the future we would be whistling in the dark if we gave a wholly reassuring answer."
But a Department of Health spokeswoman said "significant action" was being taken to improve the support given to older people.
She pointed out that all over-75s should now have a named GP to co-ordinate their care, while a Â£5.3bn pot of money - mainly from NHS funds - has been set aside this year for joint projects between councils and health services.
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A million older people in England struggling with everyday tasks, such as washing and dressing, are being left to fend for themselves, campaigners say.