Unions will be stepping up a campaign this week to get the Government to support its campaign to win rights for more than a million agency workers.
They say agency staff working in the telecoms, construction, agriculture, print, food, drink and manufacturing industries, as well as local authorities, deserve the same pay and conditions as full-time workers, and need extra protection against abuse.
Agency staff will be lobbying MPs tomorrow to ask them why they are suffering discrimination.
The workers say they earn less pay and overtime rates and are denied access to health and safety and other workplace training.
But business owners say giving agency staff the same rights as full-time workers would damage Britain's flexible jobs market and cost jobs.
The lobby is being held on the day that a Private Member's Bill by Labour MP Andrew Miller (Ellesmere Port and Neston), which aims to give agency workers the same rights as full-timers, receives its second reading.
Tony Dubbins, chairman of the Trade Unions for Labour Organisation (TULO), said: "We have been encouraged by the level of support that the campaign for legislation has gained among MPs and across the labour movement but this is further evidence of the need to act urgently to protect the growing number of agency workers from widespread abuse.
"The Labour Party committed to introduce UK legislation to protect agency workers before the last election and now we and the estimated 1.4 million agency workers in the UK.
"Now we need the Government to act on that promise by backing Andrew Miller's Bill."
Union leaders will join this week's parliamentary lobby, including Brendan Barber of the TUC, Tony Woodley of Unite and Billy Hayes of the Communication Workers Union.
But the CBI, British Chambers of Commerce, the Engineering Employers Federation and other groups have been pressing ministers not to introduce any new measures.
Tom Hadley, director of external relations at the Recruitment and Employment Confederation, said: "Whilst the protection of vulnerable workers is always foremost in the minds of agencies across the country, it is imperative that the protection of jobs and a flexible labour market is not overlooked.
"Now is not the time to bring in legislation that could potentially impact on job creation.
"Most of the instances of worker abuse already breach existing regulations and the key is to ensure that these are effectively enforced.
"It is important to recognise the contribution that temporary workers make to the labour market and also to the opportunities that temporary work provides to job seekers."