One in five workers are losing £6,000 a year by doing unpaid overtime, new figures show.
The TUC said workers put in nearly £32bn-worth of free hours last year.
People in their 40s are most likely to do unpaid overtime, with 26% putting in unpaid hours, compared to 20% of all UK workers, the figures showed.
Education was found to benefit from the most free work, with more than one million people doing unpaid overtime.
The TUC called for employers to ensure their staff were not working excessively long hours.
The increased use of zero-hours contracts shows working people are not benefiting from the economic recovery, shadow business secretary Chuka Umunna has said.
"Ministers have watered down every person's rights at work and zero-hours contracts have gone from being a niche concept to becoming the norm in parts of our economy," Mr Umunna said.
"The findings today that there are now 1.8 million zero-hours contracts and that the number of people reporting they are on a zero-hours contract for their main job has risen by almost 20% is yet another stark illustration of a recovery which is not working for working people."
Business Secretary Vince Cable said while the contracts have been abused in the past, they can provide benefits to workers as well as employers.
Business Secretary Vince Cable has said zero-hours contracts can be beneficial to workers despite admitting the controversial contracts have been abused in the past.
Mr Cable said he was looking at introducing rules which ban contracts which prevent zero-hours workers from finding employment elsewhere.
"Zero-hours contracts are valued by many employers and individuals who want flexibility in the hours they work, such as students, people with caring responsibilities and those who want to partially retire," Cable said.
"However, historically there has also been some abuse in these types of contracts.
"That is why I am taking legislation through Parliament at the moment to ban exclusivity clauses in zero-hours contracts which prevent people looking for additional work to boost their income. We want to make sure that people who are on zero-hours contracts get a fair deal."
Zero-hours contracts "sum up what has gone wrong" in the workplace, the TUC has said, after new figures revealed an increase in the number of people employed on the controversial contracts.
The TUC said zero-hours contract workers are on average £300 worse off than staff on permanent contracts, and miss out on sick pay and other protections.
"Zero-hours contracts sum up what has gone wrong in the modern workplace," TUC general secretary Frances O'Grady said.
"They shift almost all power from the worker and give it to their boss. Anyone on such a contract has no guarantee of any work from one day to another. Put a foot wrong and you can find yourself with little or no work.
"Employers often argue that they offer flexibility but try telling that to zero-hours workers who can't get a mortgage or pay their rent."
New figures have revealed that the number of zero-hours contracts have increased.
Employers say the contracts offer flexibility, but critics argue that they leave workers without financial stability.
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The number of zero-hours contracts increased from 1.4 million to 1.8 million last summer, new figures have revealed.
The figure for last August is 400,000 higher than a previously published estimate in January 2014, the Office for National Statistics said.
The number of people saying they were employed on zero-hours contracts in their main job was 697,000 in the quarter to December, up from 586,000 in the same period in 2013.
Around a third of people on zero-hours contracts want more hours, the ONS report said.
Zero-hours contracts - which allow employers to hire staff without a guarantee of a minimum number of hours - have been criticised as failing to provide enough job security or financial stability.
The ONS warned that the increase could be due to a greater awareness of the contracts.
Wonga has announced restructuring which expected to lead to the loss of 325 jobs.
The payday lender currently employs around 950 people and said the losses come as it aims to, "refocus on its consumer businesses."
Wonga can no longer sustain its high cost base which must be significantly reduced to reflect our evolving business and market. Regrettably, this means we’ve had to take tough but necessary decisions about the size of our workforce. We appreciate how difficult this period will be for all of our colleagues and we’ll support them throughout the consultation process.
325 jobs are to go at Wonga as part of "a restructuring and cost reduction programme".
The move is hoped to save the company £25 million over the next two years. Wonga said costs at the company tripled between 2012-2014.
There will now be a 30 day consultation period for those who are at risk of losing their jobs.
The Government's been celebrating new figures that show the number of people out of work has fallen to a near seven-year low.Read the full story ›
The number of migrant workers in Britain from the European Union has risen to 1.9 million - a rise of more than 10.5% year on year.
That includes those from Bulgaria and Romania, whose individual employment numbers have risen 15%.
One of the reasons for the increase is that employment curbs were lifted for citizens of the so-called A2 countries in January 2014, prompting warnings from the likes of Ukip of a surge of immigration.
According to the Office of National Statistics, around 172,000 people from the Romania and Bulgaria were working in Britain in the last three months of 2014, up 22,000 on the same period the previous year.
However, the number fell 9% quarter-on-quarter from 189,000 between July and September last year.
Eurostat also said unemployment rates were 6.4% in Romania and 10.8% in Bulgaria at the end of January.
The comparable figure for the UK was 5.9%.
Employment minister Esther McVey has called the increase in youth unemployment "a tiny blip".
Figures released by the Office for National Statistics showed youth unemployment rose by 3,000 between October and December last year.
Employment minister Esther McVey stressed that youth unemployment had fallen since the coalition government came to power and said a record number of women were in work.
"Around 1,600 people have been finding a job every day over the past year," she added.