Ministers are in a "state of denial" about poverty, a UN expert has said following a 12-day tour of the UK.
Philip Alston, special rapporteur on extreme poverty, said despite being in the one of the world's richest countries he had encountered "misery".
Levels of child poverty are "staggering" and 1.5 million people were destitute at some point in 2017, the Australian said.
The government rejected his analysis, pointing to rising household incomes.
Prof Alston, an expert in human rights law based at New York University, visited locations including Belfast, Bristol, Cardiff, Edinburgh, Essex, Glasgow, London and Newcastle on a fact-finding mission.
He met people affected by poverty as well as government officials, discussing the impact of austerity, changes to benefits and local government funding.
At a news conference in London, he said he witnessed "a lot of misery, a lot of people who feel the system is failing them, a lot of people who feel the system is really just there to punish them".
Quoting figures from the Joseph Rowntree Foundation, he said that more than 1.5 million people were destitute at some point in 2017, meaning they lived on less than £70 a week or went without essentials such as housing, food, clothing or heating.
A fifth of the population, amounting to 14 million people, are living in poverty, Prof Alston said.
How is poverty measured?
Prof Alston used a new measure by the Social Metrics Commission to come up with the figure of 14m people in the UK in poverty.
It is a measure of "relative poverty", meaning it looks at the percentage of people living with less than 55% of the median income, taking into account costs such as childcare, housing, debt and disability.
But the government has preferred to use the measure of "absolute poverty".
This counts the number of people in households with less than 60% of the median income as it was in 2010/11, so it shows how living standards of low-income households have changed over time.
By that measure, the government says there are a million fewer people in absolute poverty than there were in 2010.
Among experts in the media, think tanks, Parliament and organisations such as the National Audit Office, Prof Alston said there was "close to unanimity" that Britain was not doing enough to combat poverty.
But he said the view from ministers was the opposite.
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"Ministers with whom I met told me that things are going well that they don't see any big problems and they are happy with the way their policies are playing out," Prof Alston said.
He said the government was preoccupied with reducing welfare dependency but said he could not believe ministers were as happy with the results as they claimed to be.
Prof Alston said: "The era of connectivity and the era of social media makes it much less sustainable to have this dramatic difference between people living the high life, a higher life than has ever been lived before, and at the other end, people who can't afford a tin of beans, can't afford the seventh meal of the week."
Analysis: The Britain that Alston saw
by Michael Buchanan, BBC social affairs correspondent
Britain's poorest communities often feel ignored, that politicians aren't interested in their experiences. For a UN representative to hold public meetings, to seek out their concerns, mattered.
Some of the gatherings were packed - hundreds of people took the opportunity to be heard.
What emerged for Philip Alston was an astounding disconnect between what he heard at those meetings, backed by other research he'd done, and the discussions he held with the ministers who oversee the very system causing such hardship for many people.
Some will ask why the UN is looking at poverty in the UK - one of the world's richest nations. The answer, said Prof Alston, is that the government has supported these UN investigations in other countries, and couldn't therefore oppose the legitimacy of his visit here.
These harsh criticisms of the government's approach to the poor may not make their lives better immediately. But their daily struggles have appalled the UN, and in time, that might matter.
The approach to benefits was "punitive, mean-spirited, and often callous", Prof Alston said.
He gave the example of "draconian sanctions" which shut people out from the benefits system for weeks or months at a time, sometimes for minor infringements such as missing an appointment.
Many of the "harsh" policies could be ended "overnight" at little cost, he claimed.
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These included the delay of five to 12 weeks before Universal Credit was paid, the single household payments which give more leverage to controlling or violent partners, and the two-child limit for benefit claimants.
Prof Alston compared this limit to China's notorious one-child policy and said it was "a perfect way to punish families".
Welfare changes have disproportionately affected women, especially single parents, he said.
"If you got a group of misogynists in a room, and said guys, how can we make this system work for men and not for women, they wouldn't have come up with too many other ideas than what's already in place."
He also warned that the poor would "bear the brunt" of the expected impact of Brexit on the UK economy, and said the fall in the value of the pound had already cost low-income families £400 a year.
"In my meetings with the government, it was clear to me that the impact of Brexit on people in poverty is an afterthought," he said.
Prof Alston's report will be presented to the UN Human Rights Council in Geneva next year.
A spokeswoman for the Department for Work and Pensions said: "We completely disagree with this analysis."
She said that household incomes have "never been higher", income inequality has fallen and there are one million fewer people living in absolute poverty compared with 2010.
It is not the first time the UK has been criticised by UN special rapporteurs, who are independent human rights experts appointed for fact-finding and monitoring missions around the world.
In 2014, urban planning expert Raquel Rolnik said the "bedroom tax", which meant social housing tenants with spare rooms had to pay more rent or move somewhere smaller, undermined the right to adequate housing.
On that occasion, ministers said her report was a "misleading Marxist diatribe" and made an official complaint to the UN.