Siptu president Jack O'Connor has warned there will be no further social partnership agreements unless agency workers are granted equal rights that are enshrined in law.
Mr O'Connor said agency workers were being abused by employers, many of whom were creating conditions akin to slavery.
"The present situation is unacceptable," the Siptu leader told a meeting of union members on the issue in Waterford last might.
"We have a deplorable system of rented labour at present in which vulnerable people have effectively no employment rights in practical terms."
He said tens of thousands of workers were affected with "rogue agencies" reducing the security and quality of employment rights.
The practice impacted all workers.
"This phenomenon is sometimes defended on the grounds of protecting flexibility and meeting the competitiveness challenge. It does nothing of the kind.
"In fact, the reverse is the case, because it is deferring the day when we have to face up to the urgent need to launch a major national effort to upskill more than 500,000 workers in our economy."
He said society needed to stop thinking in terms of retraining for workers and begin working towards a system with access to third-level education for all workers.
It was only with this mindset change that people would reach their potential and the economy would be capable of meeting what he termed "the competitiveness challenge".
"Even a commitment in Towards 2016 to establish a fund to assist workers who had not been to third level with the cost of their educational fees has not yet been honoured," Mr O'Connor continued.
"There will be no further social partnership agreement unless the principle of equality of treatment for agency workers is conceded in accordance with the standards that apply within the most advanced EU countries."
Last night's meeting in Waterford city was the latest in a number of meetings organised by Siptu on the issue of securing better conditions for agency workers.
The union, Ireland's biggest, believes agency workers generally have little or no job security, have little or no access to sick pay or pension entitlements or to other non-pay benefits.
Their rates of pay are generally lower than that of the regular workforce and collective representation or negotiations has proven difficult to achieve.
