Agency workers are being denied equal pay because the Government has failed to put into practice European rules, the Trades Union Congress (TUC) claims.
The TUC, the umbrella body for the trade unions, says agency workers are still being paid less than their permanent staff counterparts. It is complaining to the European Commission about the issue, the BBC said.
Agency staff who have worked at a company for more than 12 weeks should be entitled to the same pay as permanent workers, the TUC says.
Though the Temporary Agency Workers Directive came into effect two years ago, its implementation by the Government is flawed because an exemption means that for workers directly employed by the agency, a company does not have to pay the same rate as that earned by a full-time staff member.
They do get paid for at least four weeks between assignments however, the TUC said.
It claimed there has been a large rise in such contracts, with one in six agency workers on them, especially those in low-paid, low-skilled jobs.
The TUC is calling for the banning of these contracts, the BBC said, and has asked the European Commission to investigate.
TUC general secretary Frances O'Grady said: "The recent agency worker regulations have improved working conditions for many agency workers without causing job losses. However, the regulations are being undermined by a growing number of employers who are putting staff on contracts that deny them equal pay.
"Most people would be appalled if the person working next to them was paid more for doing the same job, and yet agency workers on these contracts can still be treated unfairly."
The Department for Business, Innovation and Skills told the BBC: "We worked closely with both employers and employee organisations to successfully implement the Agency Workers Regulations.
"We will of course consider carefully any information the TUC presents to the European Commission."