Best Companies analysed data supplied by charities applying for its annual awards scheme for good employees, and found that 18 per cent of voluntary sector staff felt their job was not safe.
The figure means that the score the charities achieved in the job security category was nine percentage points below the benchmark score for 'high-performing organisations'.
A further 18 per cent of voluntary sector employees said they would leave their jobs tomorrow if they had other posts to go to.
Charities also fared poorly on staff confidence in senior management, but did well in 'wellbeing' categories such as providing a good work-life balance.
Peter Bradon, head of research at Best Companies, said the figures on job security were worrying.
Best Companies will announce this week that four voluntary sector organisations have been handed the highest three-star rating for employers.
Two organisations - debt counselling charity Christians Against Poverty and St Wilfrid's Hospice in Chichester - have won the three-star award for the first time.
Social inclusion charity P3 and Sandwell Community Caring Trust both retained the three stars they won last year (Third Sector, 7 February 2007).
The quartet were among 20 voluntary sector organisations recognised by Best Companies, which awards between one and three stars to applicants that meet its standards.
Fewer than half of more than 650 companies from across all sectors that applied for a star award were successful.