Workers on zero-hours contracts earn nearly £300 a week less than permanent employees, according to a new report.
A study by the TUC showed that average weekly earnings for zero-hours workers are £188, compared to £479 for people on permanent contracts.
The union organisation said its research also revealed that zero-hours workers are five times more likely not to qualify for statutory sick pay than permanent workers as a result of their lower level of take home pay.
Two-fifths of zero-hours workers earn less than £111 a week - the qualifying threshold for statutory sick pay - compared to one in 12 permanent employees, said the report, published to mark the start of the TUC's Decent Jobs Week.
The TUC said its campaign was aimed at drawing attention to the millions of people "trapped" on low pay and insecure work.
The study of official data from the Labour Force Survey found that one in three people on a zero-hours contract had no regular income, often with differing amounts of weekly pay compared to staff with other kinds of work arrangements.
Women on zero-hours contracts earn £32 a week less than men on average, the TUC added.
TUC general secretary Frances O'Grady said: "The growth of zero-hours contracts, along with other forms of precarious employment, is one of the main reasons why working people have seen their living standards worsen significantly in recent years.
"It is shocking that so many workers employed on these kind of contracts are on poverty pay and miss out on things that most of us take for granted like sick pay.
"While it is good to see employment is rising, if the UK doesn't create more well-paid jobs with regular hours we will continue to have a two-tier workforce where many people are stuck in working poverty.
"The increase in casual labour also helps explain why income tax revenues are falling which is not only bad for our public finances but for society too. The lack of regular hours and income makes it difficult for households to pay bills and take on financial commitments such as rents and mortgages."
The study was published ahead of the latest unemployment figures on Wednesday.
A Business Department spokesman said: "Zero-hours contracts have a place in today's labour market. These types of contracts provide important and flexible employment opportunities that suit most people in these jobs and provide an average of 22 hours work a week.
"Zero-hours contracts have been used responsibly by many businesses for many years, but unfortunately we know that some abuse does take place. This is why we are clamping down on these rogue employers.
"We are banning the use of unfair exclusivity clauses in zero-hours contracts and increasing the availability of information for employees on these contracts.
"Many individuals on zero-hours contracts are entitled to the same employment rights as permanent members of staff and in October the Business Secretary announced a review to ensure employers know their responsibilities and individuals know their rights."