Teaching assistants and other public services support staff are putting in millions of hours of unpaid overtime, according to new research.
The union Unison said its research showed the impact of years of job cuts, which had led to "intolerable pressures" across the public sector.
A survey of almost 1,000 workers in education, the NHS, local government, police and other areas found that more than two out of five were doing unpaid overtime most weeks.
Some said they were taking on the work of colleagues who had been made redundant, and some were working beyond their pay grade.
Four out of five survey respondents said they were working harder than a year ago, but a third believed they were much less productive because of cutbacks.
Teaching assistants 'undervalued'
The union said workers felt demoralised as a result of austerity, adding that morale was at "rock bottom".
Unison said cuts had been disproportionately targeted at support staff.
General secretary Dave Prentis said that support staff were "among the lowest paid in public services".
"All too often they're overlooked by politicians, despite the vital jobs they do," he added.
"It's no wonder they feel overworked and undervalued. Many are facing intolerable pressures because of cutbacks, which have triggered staff shortages.
"The government must commit to funding the jobs needed to guarantee safe, high-quality services.
"A failure to act will undermine standards further and weaken public confidence further still."