A Labour government would impose a crackdown on the use of zero-hours contracts by "exploitative" employers, Ed Miliband is to promise trade unions.
As many as 5.5 million people could be on the controversial arrangements under which workers do not know if they have work from one week to the next, according to a survey of Unite members.
It says the "staggering" figure - five times previous estimates - shows there is a "growing sub class" of insecure, low paid employees earning an average of £500 a month.
Mr Miliband will tell the TUC Congress that while zero-hours contracts are useful in some cases they have been "terribly misused" in many others.
Under his proposals employers would be banned from insisting zero-hours workers be available even when there is no guarantee of any work. Zero-hours contracts requiring people to work exclusively for one business without the guarantee of adequate hours in return would be outlawed. And there would be action taken to prevent them being used for people working regular hours over a sustained period.
The Labour leader has asked Norman Pickavance, former director of human resources at supermarket Morrisons, to consult with businesses on how the changes could be implemented and whether the law needed to be changed to enforce them.
In his address, Mr Miliband will acknowledge that more flexible working has been one of the keys of keeping people in work despite the recession - "working fewer hours, putting jobs above pay rises".
"We need flexibility," he will say. "But we must stop flexibility being used as the excuse for exploitation. Exploitation which leaves workers carrying all of the burdens of unpredictable hours, irregular pay, no security for the future. And nowhere is that more true than when it comes to zero-hours contracts. Of course, there are some kinds of these contracts which are useful. For doctors, or supply teachers at schools, or sometimes, young people working in bars. But you and I know that zero-hours contracts have been terribly misused. This kind of exploitation has to stop.
"We will support those businesses and workers that want to get on in life. But we will ban practices which lead to people being ground down. For this Conservative-led Government that is the way an economy succeeds. Insecurity for the many. Rewards just for those at the top. That's not my vision for the way an economy succeeds. I believe that what makes an economy strong is not a few people at the top, but the forgotten wealth creators: the people who put in the hours, do the work, do two jobs. We will only build a recovery that works for all, when working people feel confident and secure at work."
The study of 5,000 members of the Unite union by the social survey company Mass1 found that more than one in five were on zero-hours contracts - earning an average of £500 a month. It suggested the under-30s were more likely to be on a zero-hours contract, with half of respondents falling into the 16-30 category. Unite said the findings also pointed to employers using zero-hours contracts to avoid paying holiday pay and sick pay, with just over a third saying they do not get holiday pay and 77% receiving no sick pay. Only around one in seven said they wanted to stay on the zero-hours contracts, which the union maintained "bust a hole" in arguments that a majority of workers choose them.