Mr Brown says he does not want the plans to jeopardise jobs or business
Trade unions have given a cautious welcome to Gordon Brown's plans to introduce a law giving new rights to agency workers.
Unions have been calling for ministers to act on complaints of poor pay and conditions among agency staff.
The PM said it was not fair that agency workers can be paid less than the employed staff they work alongside.
But business leaders say the plans will put employers in an impossible position and reduce job opportunities.
The prime minister turned his attention to agency workers as he outlined his legislative programme for next year.
He told MPs the government was committed to flexibility and fairness in the workplace and would do nothing that jeopardised jobs and businesses taking on workers.
"But most people agree that it is not fair that even after months in the job, agency workers can currently be paid less than the staff they work alongside - and as a result permanent staff can feel they are being unfairly undercut," he said.
Business Secretary John Hutton will bring forward legislation "subject to an agreement between employers and employees and in Europe", he said.
This would "for the first time ensure new rules for fair treatment of agency workers here in Britain".
The unions have accused the government of blocking European legislation on the issue.
But TUC general secretary Brendan Barber said he welcomed Mr Brown's "clear recognition that agency workers get a raw deal and that there is a need to make life fairer for agency workers".
"Unions will now step up their campaign to secure proper protection and a fair deal for agency workers," he said.
Billy Hayes, general secretary of the Communications Workers Union, said he was pleased to hear "such a positive statement" coming from Mr Brown.
"There remains a lot of detail to be agreed but this is a positive step in bringing equality of rights to over a million of the most vulnerable workers and their families," he said.
But Chris Hannant, head of policy at the British Chambers of Commerce, opposed the move.
"The UK labour market's flexibility has been one of the major reasons the economy has performed strongly and created so many jobs over the last decade," he said.
"Proposals that undermine the advantages of this flexibility will not only damage employers, but will reduce job opportunities for those people that the proposals seek to protect."
Alan Tyrrell, employment chairman of the Federation of Small Businesses, said the measures proposed would put firms "in an impossible position".
"You can't have an extension of flexible working and at the same time clamp down on the means by which many small businesses cope with it, which is often through temporary workers."
Miles Templeman, director general of the Institute of Directors, said: "We remain opposed to any changes that will damage the flexibility of the UK's labour market."
The institute added that any qualification period for giving agency workers new rights must be counted in months, rather than weeks.
Mr Brown is urging unions and business groups to take part in a commission to decide on agency workers' rights.