The Citizens Advice Bureau in Scotland said fees had "altered the balance of power" between workers and employers since they were introduced in 2013.
People face fees of up to Â£1,200 for their claims to be heard by a tribunal.
The UK Ministry of Justice said fee waivers were available for those who could not afford to pay.
The Citizens Advice Bureau's Price of Justice report, published jointly with Strathclyde University, revealed the number of tribunal cases lodged during the first quarter of last year fell by 81% compared with the same period 12 months previously.
Citizens Advice Scotland (CAS) spokeswoman Lauren Wood said: "A few weeks ago we published CAB evidence showing that the number of Scots being exploited at work was increasing.
"Today we publish a new report which shows how such workers are being prevented from getting the justice and compensation they are entitled to.
"It's two years since the government introduced a fee for employment tribunals.
"The evidence shows that people have been deterred from taking their grievances to tribunal because they simply can't afford to pay the fee."
She added: "One of the most worrying aspects of this current situation is that it has altered the balance of power at work, with rogue employers now feeling they can exploit or mistreat their employees without risk of being called to account.
"In highlighting this issue today we call on the government - and all political parties - to think again about the price of justice, and to get rid of these fees.
"Justice should be available to everyone, not just the wealthy."
A Ministry of Justice spokesman said: "We want people to resolve these issues using quicker and simpler alternatives such as arbitration and mediation and only go to court as a last resort.
"We have made sure fee waivers are available for those who can't afford to pay, as well as diverting people away from potentially acrimonious hearings, where possible, through a new early conciliation scheme which has already been used by over 60,000 people in its first nine months.
"The government has committed to reviewing employment tribunal fees but believes this is better determined by the new administration following the election."

Meads, 80, played 55 times for his country, and was voted the greatest ever All Black in a national poll in 1999.
The former second row said the diagnosis has been hard on his family, and requested privacy.
"Thank you to everyone for their concern, but for now, it's about me fighting this," he said.
The introduction of fees for employment tribunals has stopped thousands of Scots from challenging rogue employers, according to a charity.
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New Zealand rugby union great Sir Colin Meads has been diagnosed with pancreatic cancer.