Millions of workers are facing insecurity, moving in and out of jobs, and poverty, according to a new report.
The Joseph Rowntree Foundation (JRF) said over six million people classed as living in poverty, were in households where people worked.
Excluding pensioners, in-work poverty now outstrips workless poverty, while 1.4 million people were now working part-time when they wanted a full-time job, an increase of 500,000 since 2009, said the report.
Spending on benefits and tax credits has never been higher, at 13% of GDP, while almost five million people have claimed Jobseeker's Allowance at least once in the last two years, said the Monitoring Poverty report, written by the New Policy Institute (NPI).
Julia Unwin, chief executive of JRF, said: "The level of in-work poverty is the most distinctive characteristic of poverty today. We need a relentless focus on fixing the labour market to ensure people have the opportunity to improve their prospects.
"More people than we can imagine will have experienced poverty since the downturn, circling in and out of insecure, short-term and poorly paid jobs. Tackling poverty requires a comprehensive strategy, but overcoming the frail jobs market, and the huge cost of outgoings on essentials that quickly eat up wages, must be the starting point."
Tom MacInnes, research director of NPI, said: "Low wages are a drag on economic recovery and cause families to struggle with the costs they face, trapping them below the breadline. Changes across five decades demonstrate poverty is not inevitable - reductions in child and pensioner poverty show that.
"But it is in-work poverty that is becoming the modern face of hardship, and at the same time support for working people is being cut. The high level of in-work poverty undermines any idea that better incentives to enter work, the centrepiece of Universal Credit, is some kind of cure-all."
A Department for Work and Pensions spokesman said: "Our welfare reforms will improve the lives of some of the poorest families in our communities, with Universal Credit simplifying the complex myriad of means-tested benefits.
"It will make work pay - by allowing people to keep more of their hard-earned money as they move into work - and directly lift hundreds of thousands out of poverty altogether. Furthermore, for people who have been dependent on benefits for years moving into work can seem a big risk, Universal Credit will reward people who choose to go back to work by ensuring that you are better off in work than on benefits for taking that risk."