Part-time workers in councils and schools are being used to fill full-time posts lost by "draconian" government cuts, according to a new report.
A study by Unison also found that unpaid overtime among part-timers has become "commonplace", with one in 10 putting in up to 10 additional hours every week for no pay.
The survey of more than 2,600 part-time workers, who make up around 60% of all employees in local government and schools - 91% of whom are women - revealed that one in five were covering the work of a redundant or vacant post as well as doing their own job.
Unison, which is holding its national conference in Brighton this week, said its research highlighted the start of a trend of zero hours contracts in councils. Unison, Unite and the GMB are balloting local authority workers for a strike in protest at a pay offer worth 1% for most employees.
Unison's head of local government, Heather Wakefield, said: "Part-time workers are the lifeblood of local government and schools and local services would collapse without them. Yet they are facing an all-out assault on their pay, conditions and hours of work.
"They are routinely used to fill full-time gaps in the workforce left by redundancies, but often with no security of contract, hours or income, and no overtime pay."
The study also showed that a high proportion of staff who previously worked more than 31 hours have seen their hours slashed. The proportion working 35-36 hours has fallen from 40% to just 1.4% since 2010. There have also been major cuts in hours for those working above 27 hours.