Plans to charge to take claims to employment tribunals will lead to "chequebook justice", TUC leader Brendan Barber warned on Friday.
Barber claimed government proposals to introduce a minimum fee will put workers off making a claim.
Employees will have to pay a minimum of £600 and possibly as much as £1,750 to have a discrimination claim heard at a tribunal, said Barber.
He will tell a conference in London: "Our priorities over the coming year must be to defend legal rights and access to justice, and why we must resist draconian government plans to charge people for using employment tribunals.
"This is chequebook justice, pure and simple, and is a profoundly regressive step. As so few discrimination claims succeed at tribunal anyway, many potential claimants, particularly those who lack the support of a union, would be put off from making a claim, giving a green light to unscrupulous employers to discriminate at will. That's something that ought to concern everyone who cares about justice, fairness and equality."
The government has launched a consultation on introducing charges, suggesting an initial fee of between £150 and £250 for a claim, with an additional fee of £250-£1,250 if it goes to a hearing, with no limit to the maximum award; or a single fee of between £200 and £600, but this would limit the maximum award to £30,000, with the option of an additional fee of £1,750 for those who seek awards above this amount.
The government said there were 218,100 claims to employment tribunals in 2010-11, a 44% increase on 2008-09, with the cost to the taxpayer rising from £77.8 million to £84 million over the same period.
Justice Minister Jonathan Djanogly said: "Currently, the UK taxpayer bears the entire £84 million cost per year of resolving other people's employment disputes at tribunals. This is not sustainable.
"We believe that people should pay a fair amount towards the cost of their case. Fee waivers will be available for people on low incomes to protect access to justice.
"Our proposed fees will encourage businesses and workers to settle problems earlier, through non-tribunal routes like conciliation or mediation, and we want to give businesses - particularly small businesses - the confidence to create new jobs without fear of being dragged into unnecessary actions."