The figures have been described as a 'national disgrace' and will fuel fresh concern about the number of elderly people suffering from malnutrition.
An independent report into more than 50 care homes in Sefton, Merseyside, found that the average institution spent an average of £27.50 a week on food, or £3.93 a day.
However, one unnamed home revealed that it spends only £15.86 a week, or £2.27 a day, for each of its residents.
This amount would have to cover the cost of up to three meals a day.
Last year the Daily Mail, which campaigns for dignity for the elderly, uncovered shocking figures showing that more than 600 care home patients died through dehydration between 2005 and 2009, and more than 150 as a result of malnutrition.
The report was compiled after the High Court ruled in November that a two-year council cap on care home fees by Sefton Council was unlawful.
The study, by consultants Laing & Buisson, found that the council pays £389 towards the care of a frail older person, but that the true cost of looking after such residents is at least £483. This means there is a funding shortfall of nearly £100 a week.
Campaigners say elderly people have been forced to make up the shortfall by going to their families for help.
Keith Wright, owner of Avarest, a small care home in Southport, said: 'You should be spending 10 per cent of your weekly fees on food. That means no less than £38.90 a week.
'We've just served lunch with a choice of plaice or haddock, steamed or battered, and some of our residents have had visitors who we've given tea, coffee and a biscuit. You can't do that for £2.27 a day.'
Last October, it emerged that one in seven nursing homes are breaking the law by failing to give elderly residents enough to eat and drink.
The worst cases found by inspectors included an underweight patient, who was assessed as being at a high risk of malnutrition and prescribed a high-calorie diet, eating just two pieces of toast in a day.
In another home a malnourished patient who had lost 10lb in a month and was supposed to be given snacks between meals was denied a biscuit even when he asked for one.
Age campaigners have warned that cuts to social care services mean that the country's elderly are facing an 'absolute crisis'.
Liz Kendall, Labour spokesman for care and old people, said: 'Older people in care homes need decent, nutritious food. This can't be done on a shoestring.
'More than £1billion has been cut from council budgets for older people since the Coalition government came to power. This is having a serious impact.'
Dr Ros Altmann of Saga said the figures were a 'national disgrace'. She added: 'Councils have to stop treating elderly people as a soft target.
'They are going to have to wake up to the fact that paying providers less than the true cost of care is illegal.'
A spokesman for Sefton Council said: 'This challenge is not unique to Sefton but is faced by authorities across the country.'
Sourced from The Daily Mail Online, 12th February 2012.