A cap on the total amount of benefits that people aged 16 to 64 can receive has begun rolling out across England, Scotland and Wales.
The cap is part of the government's overhaul of the benefits system, the biggest since the 1940s.
Key payments including jobseeker's allowance and child and housing benefit count towards the cap.
Changes to the benefit system have been spearheaded by Work and Pensions Secretary Iain Duncan Smith, who argues that too many people are trapped on benefits.
Critics say the cap failed to tackle underlying issues, such as the cost of housing and regional differences.
The benefits cap will be completely implemented by 30 September.
Once a monthThe cap, not yet law in Northern Ireland, is said to reflect the average working household income.
It has already been implemented in four London boroughs - Haringey, Enfield, Croydon and Bromley - since April.
The benefits cap applies to people getting jobseeker's allowance, child benefit, child tax credits, housing benefits and other key support from the government.
There is no cap on people who receive Disability Living Allowance or its successor, the Personal Independence Payment, as well some other benefits, such as industrial injuries benefit or a war widow or widower's pension.
"The benefit cap returns fairness to the benefits systems," Mr Duncan Smith said. "It ensures the taxpayer can have trust in the welfare system and it stops sky-high claims that make it impossible for people to move into work.
"The limit of £500 a week ensures no-one claims more in benefits than the average household and there is a clear reason for people to get a job - as those eligible for Working Tax Credit are exempt."
His department says about £90bn was paid out in benefit payments to people of working age and their families in 2009-10. It hopes the cap will save about £110m a year.
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