Limiting public sector pay to 2% risks "fatally damaging" industrial relations, the Trades Union Congress (TUC) has warned.
The TUC says public sector workers feel resentment over pay levels
It is urging ministers not to impose below-inflation pay rises on health, local government and other public sector workers.
The government is already under pressure after delaying part of this year's pay increase for the police.
Gordon Brown told MPs the decision was taken "in the national interest".
Launching its Speak Up For Public Services campaign, TUC general secretary Brendan Barber criticised the government's centralised pay target of 2% for the next three years.
He said the rationale that limiting pay increases would tackle inflation was not credible, citing research from Income Data Services showing increases in public sector pay follow inflation rather than fuel it.
In reality, Mr Barber said, a 2% rise represented a significant cut in living standards when the retail prices index (RPI) was much higher, currently 4.3%.
He argued the move would widen the pay gap between men and women, damage staff recruitment and morale, and have an inevitable impact on the quality of public services.
"The government is on a collision course with six million public servants," said Mr Barber.
"And yet the arguments for this draconian policy simply do not stand up. Public sector pay does not cause inflation, and holding it back does nothing to fight inflation caused elsewhere.
"Its only economic impact is on the living standards of public servants," he added.
The government is already under pressure over public sector pay.
It has been widely criticised for its failure to backdate a 2.5% pay rise for police in England, Wales and Northern Ireland.
The deal was decided through the independent Police Arbitration Tribunal.
But, as with other public sector workers such as nurses, the government decided to stagger payment of the award.
In contrast, the Scottish government is implementing the tribunal's award in full.
Jan Berry, the chair of the Police Federation, says officers feel "extreme anger" about their treatment by Home Secretary Jacqui Smith, who faced calls to resign over the issue.
Prime Minister Gordon Brown told a committee of MPs the decision about police pay was taken "in the national interest".
A Treasury spokesman said the government was committed to improving public services but the need to control inflation "in the current fiscal climate", meant that pay awards had to be "closely scrutinised".
"Following years of underpaying public sector workers, the investment this government has made has significantly improved the position of public sector careers and the salaries of public sector workers," the spokesman said.
"Our focus must now shift into better use of existing resources and we must not put the result of this investment at risk by allowing unaffordable pay growth."