Almost 600,000 poor families are facing a second year of above average council tax rises, according to new research.
The Joseph Rowntree Foundation (JRF) said 580,000 families in England will pay an average of £149 a year more than 12 months ago, having a "significant impact" on their finances.
The research group said a study of Council Tax Support, brought in last year, found that from this week 70,000 poor households will pay council tax for the first time, facing average bills of £114.
Over 6 million people classed as being in poverty live in households where people work, according to a new report.
Excluding pensioners, in-work poverty now outstrips workless poverty, say the Joseph Rowntree Foundation (JRF).
The Monitoring Poverty report, written by the New Policy Institute (NPI), also found that spending on benefits and tax credits has never been higher, at 13% of GDP.
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."