Gordon Brown has met union leaders to try to settle their dispute with employers over giving increased rights to agency workers.
More than one million people in the UK are employed through agencies
The prime minister wants them to sign up to a new commission to investigate the issue.
The unions say this is a delaying tactic and the government is reneging on previous agreements.
Last week Labour MPs supported a backbench bill to improve agency workers' rights.
The unions say the majority of workers who sign up with agencies do so because they have no choice.
More than one million people are employed via agencies, which means they do not get benefits such as sick pay.
Unions say they deserve the same pay, overtime and holiday benefits as full-time employees.
But employers' groups such as the Confederation of British Industry (CBI) say that extending their rights will compromise labour market flexibility and ultimately cost jobs.
Mr Brown wants the two groups to solve their differences at a specially convened commission, which the CBI has signalled its willingness to join.
But the unions want the government to throw its weight behind a back-bench bill on agency workers' rights which gained strong backing last week from Labour MPs.
MPs voted by 147 to 11 - a margin of 136 - to back Labour MP Andrew Miller's bill, proposing to give agency workers the same rights as permanent staff.