Bad bosses are trying to deprive workers of sick pay, national insurance contributions and even the minimum wage, research by Citizens Advice has revealed.
The charity has urged workers to seek help and keep any evidence of attempts to deny them their rights.
The most common complaints include not being paid for travelling between clients, pregnant workers having their hours cut, or being told to go self-employed.
Citizens Advice chief executive Gillian Guy said: "Bad bosses are denying people their rights, often for their own gain.
"People with complex work arrangements or those whose circumstances change can be unsure about their rights, with unscrupulous employers using the opportunity to mislead them about how they should be treated.
"Anyone who is being refused pay and protections should seek advice straight away, to help them clarify their rights and how they can raise the issue with their boss.
"For people who can't reach a resolution with their employer, it can be hard to work out who to report the problem to and what to do next."
Citizens Advice wants the Government to create a work authority to make it easier for people to get the rights they are entitled to by clamping down on illegal business practice.
The charity recently revealed that half of people on zero-hours contracts and two in five temporary workers wrongly thought they were not entitled to holiday pay.
Around 180,000 people contacted Citizens Advice over a work-related problem in the year to April.