The General Secretary of Mandate Trade Union has said zero hour and "if and when" contracts are "contracts of a century ago" that have workers "dancing on a string."
Speaking on RTÉ's Morning Ireland, John Douglas said tens of thousands of workers, across dozens of sectors, are signed to these "exploitative" contracts.
Mr Douglas said some employees do work constantly but they are aware that if anything happens or they have a run in with a manager their hours could be cut drastically.
"These are exploitative type contracts," he said.
"They demand maximum commitment from employees but minimum commitment from employers.
"They constantly have you dancing on a string that they can reduce your hours, or increase your hours, or change your days or your starting and finishing times at a week’s notice basically," he added.
"You've never any certainty and you can't apply for a second job because you don't know what your availability is like.
"It causes problems with your own family and personal life. So, it's really horrendous and puts a lot of stress on workers."
Mr Douglas called on the Minister for Social Protection to introduce robust legislation to protect workers, adding that some of the current proposals are "full of holes."
He said legislation is badly needed but it needs to have narrow bands.