The Joseph Rowntree Foundation (JRF) said Britain is leaving the EU with half a million more children "trapped" in poverty than five years ago.
A fifth of the population, over 14 million people, are in poverty, suggested the charity, in its annual state of the nation report.
Over half of those classed as in poverty are working age adults, 4.1 million are children and 1.9 million are pensioners, the study suggested.
In-work poverty has been rising faster than employment, especially among parents, the research indicated.
Child poverty has been rising since 2011/12, much faster than expected based on population growth, said the JRF.
Nearly half of children in lone-parent families live in poverty compared with one in four of those in couple families, while lone parents are more likely to be low-paid than parents in couples, according to the report.
Campbell Robb, chief executive of the JRF said: "We are seeing a rising tide of child poverty as more parents are unable to make ends meet, despite working.
"This is unacceptable. It means more families are trapped in impossible situations, struggling to pay the bills, put food on the table and dealing with the terrible stresses and strains poverty places on family life.
"It's time for us to decide what kind of country we want to be. As we leave the EU, we must tackle the burning injustice of poverty and make Britain a country that works for everyone.
"We can do this by taking action on housing, social security and work to loosen the constraints poverty places on people's lives.
"We have an opportunity to fix this and ensure everyone can reach a decent standard of living - it is one we must seize to make the country work for everyone after Brexit."
JRF quoted Hazel Ratcliffe, a working lone parent from Fife, who said: "Life can feel like a hamster's wheel - I am working and pushing myself so hard, but feel like I'm stuck.
"Most weeks I manage, but it involves rigid meal planning, then going around the supermarket with a calculator to ensure I stay within budget."
Despite being one of the richest economies in the world, today's @jrf_uk report shows there are millions of British children growing up in poverty, and that in work poverty is rising. Govts have reduced child poverty in the past and have a moral duty to do the same again. https://t.co/saoX9hKSMw
— Children's Commissioner for England (@ChildrensComm) December 4, 2018
Margaret Greenwood, shadow work and pensions secretary, said: "There is something seriously wrong when the number of people in work in poverty is increasing faster than employment."
Tess Lanning, director of the Living Wage Foundation, said: "It's incredible that millions of working parents are struggling to keep their heads above water, trapped in low pay jobs with little chance of progression.
"But as the report suggests, employers can play their part by paying workers a real living wage that covers the cost of living."
A Department for Work and Pensions spokesman said: "We disagree with this report, and there are now one million fewer people living in absolute poverty since 2010, including 300,000 children.
"With this Government's changes household incomes have never been higher, income inequality has fallen, taxes are down for families and businesses, and there are fewer children in workless households than ever before, boosting their prospects in life."
The Rt Revd Paul Butler, Bishop of Durham, said: "In a just and compassionate society, we believe that children should be at the heart of the political agenda, not side-lined by politicians who think they have more urgent matters to attend to.
"We can no longer ignore the reality that child poverty is rising and will continue to rise unless urgent action is taken.
"The sharp rise in poverty among working families with children is particularly concerning."
Alison Garnham, chief executive of the Child Poverty Action Group, said: "The evidence that child poverty is rising is undeniable but the question is, do we have a Government that is willing to see the problem and act on it?"
