The number of workers doing unpaid overtime in the public sector has increased over the past decade, with one in four putting in at least an extra hour a week, according to a new study.
The rise has almost entirely been driven by 250,000 more women working extra hours for free, said the TUC.
In 2003, more men than women did unpaid overtime in the public sector, but the position has been reversed, the research found.
Around one in six staff in private firms worked extra hours for no pay, a figure which has largely remained unchanged over the past decade, although the amount of unpaid overtime has increased.
TUC general secretary Frances O'Grady said: "Times are tough for public sector workers. As the cuts bite and fewer staff find themselves having to take on more work, unpaid overtime inevitably grows.
"Some of the increase will be down to the professionalism and commitment of staff who want to provide decent services.
"But there is also evidence of bullying and excessive management pressure in some workplaces.
"It is not surprising that morale is so low across the public sector. Hours are up, workload has increased, pay has been frozen, pensions cut and jobs insecure as public sector staff know that 60% of the cuts are still to come."
The report was published ahead of Work Your Proper Hours Day on Friday, the day when those who do unpaid overtime would start to get paid if they did all their unpaid work at the start of the year.