The Joseph Rowntree Foundation (JRF) said low pay and part-time work had prompted a sustained and unprecedented fall in living standards.
Just over half of the 13 million people living below the poverty line - set at 60% of the median or mid-point income - were from families where someone worked.
Average incomes have fallen by 8% since their peak in 2008. Some two million people have an income that is above today's poverty line, but would have been below the poverty line in 2008.
Of those in work, the number paid below the living wage rose from 4.6 million to five million in 2012. Half of working families in poverty have an adult paid below the living wage, the JRF said.
Julia Unwin, chief executive of the JRF, said the research showed that millions of people were moving in and out of work, but rarely out of poverty itself.
She said: "Hard work is not working. We have a labour market that lacks pay and protection, with jobs offering precious little security and paltry wages that are insufficient to make ends meet."
A total of 6.7 million "working families" are below the poverty line, compared to 6.3 million retired and unemployed.
The annual Monitoring Poverty and Social Exclusion report shows a very high proportion of children in poverty in the major cities. Glasgow, Manchester, Liverpool, Newcastle and parts of London are all in the 10 areas with the highest rates of poverty.
But child poverty was at its lowest level for 25 years and the number of pensioners in poverty was at a 30-year low.
The report also reveals that of the 10 local authorities where male life expectancy is below 75, seven are in Scotland. The others are in England's north-west. Almost every local authority where male life expectancy is over 81 is in the south of England.
All of Scotland except for one local authority is below the national average for life expectancy, and Glasgow stands out, with a life expectancy of 71.6, almost seven years below the national average.
The gap between Glasgow and Manchester, the English local authority with the lowest life expectancy, is 2.1 years.
The report's authors found working adults without dependent children were the most likely to be living in poverty.
The foundation found a number of positive changes, including an improvement in the labour market with falling unemployment and underemployment and, over the longer term, improvements in health and education.
Median incomes in the UK in 2011/12 are just below what they were in 2001/02 in real terms - £367 a week compared with £368.
A Department for Work and Pensions spokesman said: "Despite claims to the contrary, work absolutely remains the best route out of poverty - children in workless families are around three times more likely to be in poverty than those in working families.
"Our welfare reforms are designed to further increase work incentives and improve the lives of some of the poorest families in our communities."