A fresh row over the treatment of workers has been sparked after official figures showed the number on zero-hours contracts has increased by almost a fifth to 744,000.
People on a zero-hours contract in their main job represented 2.4% of those in employment in April-June, compared with the same period last year, the Office for National Statistics said.
The ONS estimated there were around 1.5 million contracts that do not guarantee a minimum number of hours, confirming that many workers are on more than one zero-hours contract.
The figure, for January, is 91,000 higher than the same month last year.
The new figures fuelled fresh controversy over the use of the contracts, under which employees do not know how much work they have from one week to the next.
Research published by the TUC shows that average weekly earnings for zero-hours workers are £188, compared with £479 for permanent workers.
Two-fifths of zero-hours workers earn less than £111 a week - the qualifying threshold for statutory sick pay - compared with one in 12 permanent employees.
TUC general secretary Frances O'Grady said: "Zero-hours contracts are a stark reminder of Britain's two-tier workforce.
"People employed on these contracts earn £300 a week less, on average, than workers in secure jobs.
"I challenge any minister or business leader to survive on a low-paid zero-hours contract job, not knowing from one day to the next how much work they will have."
The ONS said people on zero-hours contracts are more likely to be women, in full-time education, or older and younger workers.
Someone on one of the contracts usually works 25 hours a week.
The report said two out of five people on a zero-hours contract wanted more work, usually in their current job.
A Business Department spokesman said: "Zero-hours contracts have a part to play in a modern, flexible labour market.
"For workers such as students and those with caring responsibilities they provide a pathway to employment, particularly when the individual cannot commit to regular hours.
"However we have acted to ban the use of exclusivity clauses in these contracts which prevent people from boosting their income when they have no guarantee of work.
"This is giving working people the freedom to take other work opportunities and more control over their work hours and income."
Unite assistant general secretary Steve Turner said: "The continued rise of zero-hours contracts underlines how Government claims to be on the side of working people are nothing more than lip service.
"These figures are the tip of an insecure iceberg - they do not include short-hours contracts and the wider rise in insecure, precarious work across the economy."
Chuka Umunna, shadow business secretary, said: "These stark figures show that the Tories are the party of insecurity at work.
"Zero-hours work is on the rise with the total number of contracts rising to 1.5 million and the number of people reporting their main source of employment as a zero-hours contract having risen by almost 20% since last year. At the same time, there are now over 1.2 million people working part-time because they could not find full-time work - 200,000 more than when the Tories took office in 2010.
"Ministers are watering down vital protections at work and have refused to act to protect workers on zero-hours contracts. As long as ministers are happy to sit aside and encourage the proliferation of insecure work, more and more people won't have the security of knowing where their next pay cheque is coming from or being able to plan ahead."
James Sproule, director of policy at the Institute of Directors, said: "Zero-hours contracts offer businesses and employees an important degree of flexibility. For skilled professionals, arrangements helped preserve jobs during the downturn and protected the UK from double-digit rates of unemployment.
"As businesses began to create jobs at a record pace, attention on the quality of those jobs and concerns around zero-hours contracts boomed. This helped make sure practices like exclusivity clauses - something which run contrary to the very flexibility zero-hours contracts were designed for - were stamped out."