The Health & Safety Executive (HSE) highlighted the case in Tewkesbury as an example of over-caution. It said health and safety was given "as the excuse" for insisting volunteers obtained the insurance, which they argued was too expensive.
"It is not at all clear what risks they are insuring themselves against other than a vague possibility of civil litigation," said the HSE.
"Given the very low risks involved in maintaining the public gardens it is hard to understand why the council is insisting on the volunteers taking out insurance or not simply offering to provide the cover for the volunteers."
Last year's town mayor Chuck Pavey said taking out public liability insurance would be unaffordable. But Tewkesbury Borough Council director of resources George Hill said: "The work is on council-owned land, which means the council could be held responsible if an accident occurs."