Tens of thousands of vulnerable workers are being exploited by rogue employers, Citizens Advice has warned.
Citizens Advice Bureaux dealt with more employment queries last year
It wants the government to set up a "fair employment commission" to safeguard workers' rights and hold bad bosses to account.
The charity says gaps in the current system let some unscrupulous companies act with "near impunity".
The government says it is "determined" to tackle rogue bosses and already has a number of initiatives under way.
A joint report from Citizens Advice and Citizens Advice Scotland warns that exploitative practices such as the non-payment of tax and national insurance put good employers at a competitive disadvantage.
Last year Citizens Advice Bureaux across the UK dealt with more than half a million employment related queries.
It estimates 60% of these involved the denial of statutory workplace rights such as the minimum wage, paid holiday and sick leave and pay.
Some workers were also required to work excessively long hours or were denied proper rest breaks, the report said. Others were summarily dismissed for being pregnant.
Other high risk groups include migrant workers and those who because of age, disability of lack of skills, would struggle to find another job, it added.
Lack of protection
Four bodies are currently responsible for enforcing workers' rights - HM Revenue and Customs, The Employment Agency Standards Inspectorate, the Health and Safety Executive and the Gangmasters Licensing Authority.
Citizens Advice argues their combined remit is not sufficiently comprehensive, and leaves many workers unprotected.
"While the vast majority of employers try hard to meet their legal obligations, our evidence shows that there are still far too many rogues out there, flouting the law, ignoring rules without sanction and profiting from vulnerable workers," said chief executive David Harker.
Mr Harker said he welcomed the government's increasing recognition of the problem, but that more action was needed.
He called for the establishment of a fair employment commission with sufficient legal powers and resources to secure employees' rights and ensure bad employers are punished.
"Tens of thousands of workers in the UK economy are currently failing to benefit from the UK government's very welcome policy programme, revealing a real need for a 'fair employment commission' to give exploited workers somewhere to turn, give employers a level playing field, and root out the rogue employers," he added.
The government said a number of initiatives were already underway in this area.
"We are determined to crack down on rogue employers who prey on vulnerable people," said a spokesman for the Department for Business, Enterprise and Regulatory Reform (DEBRR).
The government has already announced tougher penalties for employers who fail to pay the minimum wage, including potentially unlimited fines for the most serious offenders.
In addition, he said two pilot projects would help boost workers' knowledge of their rights and entitlements.
"We have also established the vulnerable workers enforcement forum, due to report back next year, which brings together employers, unions and all the enforcement bodies to find better ways to stop abuses of vulnerable workers," he added.