Employers are being accused of undercutting wages by keeping people on agency contracts for years.
A study by the TUC found that three out of five agency workers are being employed in the same role at the same workplace for more than a year, with one in six in the same role for more than five years.
The TUC said that many of these workers' employers are deliberately paying them less than their permanent colleagues - a difference of £1.50 an hour on average.
At some workplaces the difference is £4 an hour or £7 for those working anti-social shifts, said the union organisation.
The industries with the highest numbers of long-term agency workers were banking and finance, distribution, hotels and retail and manufacturing, the analysis of the Labour Force Survey discovered.
The report said young workers were particularly at risk of being "trapped" in insecure agency work.
Two-fifths of agency staff employed for more than a year are aged 16-35.
TUC general secretary Frances O'Grady said: "Employers are keeping people on agency contracts to drive down wages.
"Two people working next to each other, doing the same job, should get the same wage.
"But bosses are exploiting a loophole in the law that allows them to pay agency workers less.
"The Government must scrap this loophole now - it's an undercutters' charter."
A Business Department spokesman said: "The Government recently set out plans to ensure millions of workers, including agency workers, will benefit from enhanced rights and protections.
"Under the package of proposals, all 1.2 million agency workers will be able to request a more stable contract and receive a clear breakdown of their pay.
"We are also considering repealing laws allowing agencies to employ workers on cheaper rates."