The government is obstructing a draft European Union (EU) law aimed at providing equal pay for temporary workers, according to trade unions.
The Trades Union Congress said UK objections were holding up a final agreement on the measure in Brussels.
The government argues that temps should only receive equal pay rights when they have been in the job for one year.
The TUC claims that this penalises the 800,000 UK workers who never stay in the same job for more than a year.
"The UK are keeping this issue off the table and delaying the decent treatment of agency workers," said TUC General Secretary Brendan Barber.
"There is no solid argument for denying agency temps fairness at work from their first day in the job."
He said research suggested that businesses employed temps because of the flexibility they provided, not because they were cheap.
The TUC cited a survey showing that 74% of agency temps work in the same job for less than a year.
For temps under 25 years of age, the proportion rises to 93%.
But the government denied that it was trying to block agreement on the proposal.
"Our key concern is to ensure that any directive protects agency workers without putting their jobs at risk."
British business lobbies have protested against the EU directive, arguing that it would deter employers from taking on temporary staff, destroying thousands of jobs.
The UK is thought to have the highest number of temporary workers in Europe.