One in five UK households borrowed money or used savings to cover food costs in April, a Which? survey says.
It suggests the equivalent of five million households used credit cards, overdrafts or savings to buy food.
The consumer group tracks the spending habits and behaviours of 2,000 people every month.
Which? boss Richard Lloyd described the findings as "shocking". The government said tax and benefit changes meant working households were now better off.
The figures come despite official statistics last week showing that personal insolvencies had dropped to their lowest levels in five years.
The Which? monthly tracker involves researchers interviewing a cross-section of the population online. The results can then be filtered by age, income, gender or region.
Of the one in five households borrowing or dipping into savings to pay for food, most were low income families - half of whom earned less than £21,000 a year.
Average household earnings in 2011 were about £37,000, according to the most recently available data from the Office for National Statistics.
Among the group who used savings or credit to pay for food:
- Eight out of 10 (82%) worried about food prices
- More than half (55%) said they were likely to cut back on food spending in the next few months
- Nearly six out of 10 (57%) said they found it difficult to cope on their current income
- A third (32%) borrowed money from friends and family in April
A typical weekly food bill averages about £76, Which? researchers said, up 4% on last year.
Of all the people polled, the research showed:
- A quarter said they were living comfortably on their incomes
- More than a third - 36% - felt their finances were under pressure
- Almost one third - 31% - cut back spending on essentials last month, and were most likely to be women aged between 30 and 49.
'Mixed economic picture'
Mr Lloyd, Which? executive director, said: "Our tracker shows that many households are stretched to their financial breaking point, with rising food prices one of the top worries for squeezed consumers.
"It's simply shocking that so many people need to use savings or credit to pay for essentials like food."
BBC business correspondent Joe Lynam said the economic picture in Britain was decidedly mixed these days.
"It's true that millions are at what Which? describes as 'financial breaking point', yet retail spending is growing, as are house prices, while the number of people in work is at a record high."
He added that average real incomes in Britain had fallen to the same levels as a decade ago because salaries were not rising but the cost of living was.
"The good news is that the economy is recovering, albeit at a glacial pace. The bad news is that it's not happening quickly enough for millions who are genuinely struggling to make ends meet," he said.
A spokesman for Oxfam said millions of people were under pressure from a combination of rising prices and stagnant incomes - with their problems added to by cuts to services and safety nets.
Mary Creagh, Labour's shadow environment secretary, said the UK was facing a "growing epidemic of hidden hunger".
"Families face a cost of living crisis and are being forced into debt or to use their savings simply to put food on the table.
"This incompetent government needs to wake up to the human cost of their failed economic policies and change course now," she added.
A government spokesman said nine out of 10 working households would be better off as a result of last month's changes to the tax and benefit system - with the average working household better off by more than £300 a year.
"The economy is healing: the deficit is down by a third, over 1 million private sector jobs have been created and interest rates remain low," he added.