A survey by Unite found that 43 per cent of those who responded said they had been bullied over the period. This compares with a national average of 10 per cent.
The union said the finding has forced it to renew calls for the sector to tackle "unacceptable" levels of bullying. The call is part of Unite's existing Dignity at Work campaign (Third Sector, 10 October, 2007).
"Bullying is one of the biggest issues blighting workers in the not-for-profit sector," said Rachael Maskell, national officer for the community and not-for-profit sector at Unite. "Organisations should implement a zero-tolerance approach to bullying.
"The only way for organisations to tackle the issue is by working with employee representatives to find joint ways to challenge workplace bullies and all forms of unacceptable behaviour and the underlying culture that allows them to thrive."
A lack of proper human resources structures and poor management training in the sector contributed to the problem, according to the union.
The figures came as the union gathered representatives from across the sector to look at the problem at a conference in London on 3 July.
Unite will be publishing guides for union reps working in the sector, offering advice on how to deal with members who are being bullied.