Labour leader Ed Miliband has accused Sports Direct of using Victorian practices for hiring thousands of workers on zero-hours contracts.
In a speech, he said the firm was a "terrible place to work". Labour claims 17,000 of its 20,000 UK employees are not guaranteed regular hours.
Mr Miliband promised to ban "the exploitation of zero-hours contracts".
Sports Direct, one of the UK's biggest employers, said it was continuing to review "core employment procedures".
Zero-hours contracts do not guarantee regular work for employees. Sick pay is often not included although holiday pay should be, in line with working time regulations.
The BBC's political correspondent Chris Mason said research conducted by Labour concluded that 17,000 of the company's 20,000 employees in the UK were hired on the controversial contracts.
Speaking to the West Midlands Labour Party conference in Coventry, Mr Miliband took aim at what he calls "a zero-zero economy - of zero-hours contracts and zero tax for those at the top".
Mr Miliband pledged that, under a Labour government, "if you work regular hours you will have a legal right to a regular contract".
Business and Enterprise Minister Matthew Hancock insisted that the government was already taking action.
"We're already tackling the abuse of zero-hours contracts - after 13 years of Labour doing absolutely nothing about it," Mr Hancock said.
He also accused some Labour councils of continuing to use the contracts.
Zero-hours contracts, or casual contracts, allow employers to hire staff with no guarantee of work.
They mean employees work only when they are needed by employers, often at short notice. Their pay depends on how often they work.
Some zero-hours contracts oblige workers to take the shifts they are offered; others do not.
Sick pay is often not included, although holiday pay should be, in line with working time regulations.
Focusing on the high street sports chain, the Labour leader said Sports Direct "has predictable turnover, it has big profits but, for too many of its employees, it is a terrible place to work".
"We cannot go on with an economy that allows businesses to use zero-hours contracts as the standard way of employing people month after month, year after year."
"These Victorian practices have no place in the 21st Century."
Sports Direct did not comment on Mr Miliband's remarks, but highlighted a recent statement in which the firm set out changes it was making.
A spokesman said: "The company will continue the process of reviewing, updating and improving our core employment documents and procedures across our entire business beyond its existing compliant framework."