Labour has warned that low pay and insecure work "are endemic in our economy" after new figures showed 210,000 people claiming universal credit are in work.
Some 540,000 are now claiming universal credit, a flagship policy in the Government's welfare reform programme which is being gradually rolled out across the country.
Around 39% of those are in employment but on low incomes, with the benefit paid to those in employment as well as those out of work.
Ministers say universal credit makes work pay by supplementing incomes and simplifies the benefit system.
Debbie Abrahams, shadow work and pensions secretary, said: "The Tories' principle that work will always pay under universal credit has failed.
"The increasing numbers of working people in receipt of universal credit show just how many workers are forced to rely on the social security system to make ends meet.
"Low pay and insecure work are endemic in our economy."
Universal credit combines benefits such as jobseeker's allowance and employment and support allowance, as well as housing benefit and tax credits, into a single monthly payment.
A report by charity Citizens Advice last week called for the universal credit rollout to be paused, citing "significant problems" with the system.
Research by the charity suggested many claimants fell into debt waiting for their first payment, which takes six weeks to process, while Citizens Advice also raised concerns over universal credit's administration.
The charity believes that by 2022 more than seven million households will receive universal credit, 54% of which will have someone in work.