Charity workers, including volunteers in soup kitchens, have made a plea to be treated with dignity after a study revealed they were bullied, teased and criticised.
Unite said a survey of more than 300 workers showed that two out of five had faced bullying at work, often left feeling humiliated and undermined.
People giving up their free time over Christmas to answer calls to crisis telephone hotlines or work in animal shelters were among those complaining of problems.
Around one in five of workers surveyed by the union said they had taken time off because of bullying and some had even moved jobs because of the problem.
Unite's national officer Rachael Maskell said: "The survey results are shocking.
"Charity workers give of themselves all year round to help others and often give up their Christmas and New Year.
"It is outrageous that employers have let so much unacceptable behaviour, bullying and harassment go on.
"Bullying wrecks work and family as well as destroys those on the receiving end. This has to be tackled.
"Unite will not cease campaigning until we are assured that all organisations have taken a comprehensive approach to promoting dignity at Work.
"Unite will not tolerate unacceptable behaviour from bosses or anyone else. There is no place for bullying in the workplace."
