Just 14% of valid claims are being taken forward by people at employment tribunals following the introduction of fees to pursue a claim, new figures show.
Citizens Advice said that since the Government introduced fees of up to £1,200 last July there has been a 73% drop in claims compared to the same period the previous year.
Despite this, millions are still logging onto Citizens Advice's website in need of help with employment matters, up 42% on last year, while tribunal searches have increased by 54%.
The charity is calling for a review of the system.
Its analysis found that four out of five cases had a 50/50 chance or higher of success if they were pursued to tribunal, but less than a third (31%) of the potential success cases were likely or definite to proceed to tribunal.
Less than a quarter of claims worth £1,000 or less are likely to be, or are definitely being taken forward, with the cost said to deter people in just over half the cases.
Unfair dismissal and withholding wages were the most common issues for people thinking of bringing a claim forward, along with holiday pay.
The complexity of the process, stresses involved and fear of losing their job also dissuaded potential claimants.
Gillian Guy, chief executive of Citizens Advice, said: "Employers are getting away with unlawful sackings and withholding wages.
"People with strong employment claims are immediately defeated by high costs and fees.
"The risk of not being paid, even if successful, means for many the employment tribunal is just not an option.
"The cost of a case can sometimes be more than the award achieved and people can't afford to fight on principle any more.
"Citizens Advice wants to see a fair and robust review of the employment tribunal system to make it work for all people and employment abuses eradicated."
Justice Minister Shailesh Vara said: "It is in everyone's interest to avoid drawn out disputes which emotionally damage workers and financially damage businesses. That's why we are encouraging quicker, simpler and free alternatives such as the Early Conciliation service provided by Acas.
"It is not fair for the taxpayer to foot the entire £74 million bill for people to escalate workplace disputes to a tribunal. And it is not unreasonable to expect people who can afford to do so to make a contribution. For those who cannot afford to pay, full fee waivers are available."