The Adult Learning Inspectorate also criticised the leadership and management of the Workers' Educational Association, which last year enrolled 150,000 learners.
The inspection, the largest yet undertaken by ALI, involved 59 inspectors visiting nearly 600 locations throughout England. They spent 612 days interviewing almost 5,000 learners and 555 staff.
Now, the inspectors have devised a series of follow-up visits to the WEA over the next 12 months to discover if the association can use the inspection report as a springboard for improvement.
The WEA offered more than 13,000 courses to adults across England last year. More than 90 per cent of its pound;32.3 million annual budget was funded by the Learning and Skills Council.
David Sherlock, ALI's chief inspector, said the report would provide the WEA with "a true picture of what exactly is happening throughout the organisation".
He added: "It gives them an opportunity to decide on the changes that need to be made. The aim of inspection is to drive up standards to ensure that every learner receives a positive and worthwhile learning experience."
The inspectors say the quality of WEA provision "is not adequate to meet the reasonable needs of those receiving it". It described provision in visual and performing arts, media, English, languages, and communication and foundation programmes as unsatisfactory. It said the association's approach to quality assurance was "very weak".
However, its community development programmes were graded as good, and several curriculum areas, including science, mathematics, and ICT, were graded as satisfactory.
The WEA is a registered charity. Founded in 1903, it became a single association comprising its previous 13 districts and Scottish associations in 1993.