Stephen Dorrell said patients with conditions such as dementia used to get free care in NHS geriatric hospitals.
But the number of places has fallen by nearly 80% in the UK over the past 20 years - despite the ageing population.
He said this had pushed people into the means-tested social care system where they were often charged for treatment.
In an interview with the BBC, he said the redrawing of the boundaries had been allowed to creep in without proper debate or scrutiny and urged politicians to face up to the issue.
An expert commission has already been set up by the government to look into the issue of social care funding in England.
But Mr Dorrell was speaking about a specific group of patients whom he believes the NHS has turned its back on.
As well as dementia patients this includes people such as stroke victims and those with Parkinson's disease who struggle to get the NHS to pay for medical treatment they receive.
Mr Dorrell, who was health secretary towards the end of John Major's time as prime minister, said: "People are being charged for care that they would have got free from the NHS 20 or 30 years ago.
"In effect there has been a change in the definition of what constitutes NHS care and that has happened without proper debate.
"Unfortunately, it has been ignored because both politically and financially it is tricky for politicians to face up to it. But because it has not been done in a planned way there is great unfairness in the system. We see examples of cost shunting and bureaucracy that cause individuals problems.
"I would not want to see a return to the old system of geriatric hospitals - care is much better now - but you have to question whether it is fair that this group of people are being charged in this way?"
Evidence on the changes to the nursing care home and geriatric hospital sectors lend support to his view.
Figures from analysts Laing and Buisson show that the number of geriatric beds fell from more than 80,000 in 1988 to 16,300 last year.
During the same period nursing home places more than doubled from 78,300 to 179,400. On top of that there are now nearly 300,000 residential care places, although these are less likely to have patients with severe medical conditions.
NHS funding is available to people with the most severe medical needs who are in care homes or living independently under a system called continuing care.
But campaigners claim the funding is too bureaucratic and difficult to qualify for. Little over 50,000 people currently get it.
Age UK policy adviser Stephen Lowe agreed Mr Dorrell was right to highlight the issue, saying the NHS had "unilaterally retreated" from its responsibilities.
Ruth Sutherland, interim chief executive of the Alzheimer's Society, added: "There are hundreds of thousands of people missing out on valuable financial help because they don't 'tick the right boxes'.
"People with dementia are some of the hardest hit by this deliberately tricky system. They have complex physical health needs which should often be covered by the NHS but arguments over funding see them denied this care.
"Instead they are forced to pay for social care which adds huge financial burden to people already under emotional and physical strain. Faced with exhaustion, they may not have the strength to challenge being turned down."
A Department of Health spokeswoman acknowledged there had been problems with people accessing NHS funding, but said the situation was now improving following new guidance to NHS trusts.
She added that recent data showed there had been an overall increase in the numbers of people getting continuing care funding.