More than 80% of clients seeking help from Citizens Advice say they would be put off going to an employment tribunal by the level of fee they would face.
A poll of 361 clients of the advice service found the majority are scared off by the fees regime that was introduced in July 2013.
The government has said the fees put off weak or vexatious claims and help to fund the tribunal system.
But in research published by Citizens Advice, the organisation said the fees are likely to cover just 7% of the cost of running tribunals.
The number of cases won by claimants has actually fallen in the past two years to below 60%, suggesting that fees may have had a bigger impact on strong cases than weak ones.
Current fees are set at £390 for type-A cases (such as unpaid wages or notice pay) and £1,200 for type-B cases such as unfair dismissal or discrimination.
There is a system of remissions for claimants on benefits or low incomes, but the poll found just 29% of clients were aware of this help.
The research found the fees were high in relation to the money that clients took home. The poll revealed, based on declared salaries from respondents, that 47% of type-B claimants would have to put aside all of their discretionary income for six months to save the £1,200 fees.
Citizens Advice called for tribunal fees to be reduced immediately and aligned with county court fees to widen access.
It also wants the Ministry of Justice to do more to promote awareness of the remissions system, and make it easier for potential claimants to work out whether they are eligible for fee remissions. Currently, Citizens Advice pointed out, claimants must read a 32-page PDF to check their eligibility.
The advice service also wants the government to commission research into the number of spurious claims and what can be done to deter them.
The report added: ‘Currently the system is imbalanced against claimants, and balance cannot be properly struck without knowing what puts off weak and genuine claims.
‘The research should seek to establish what proportion of ET claims can be considered unjustified and what measures could be taken to protect employers from these without deterring legitimate claims.’
The legal battle over the introduction of the fees is set to continue this year.
The High Court last month rejected a challenge from trade union Unison, in effect because the union could not produce sufficient evidence to show that fees are making it more difficult to bring cases to tribunal. Unison has said it will appeal.