The government wants the adult population to increase its skills
Thousands of adults targeted by government skills-boosting schemes have dropped out of evening classes, a survey of 5,000 people suggests.
Those in the lowest income groups have been hit the hardest, with the share of skilled manual workers on courses falling by a fifth to 33% in one year.
This reverses the participation gains of the last 12 years, the figures show.
Young adults have also been hit hard, with a 16% fall in the number of 25 to 34-year-olds on courses in 2007-08.
Any small rises in the share of unskilled, unemployed and retired people on adult learning courses since Labour came to power have also been reversed.
Just 26% of social groups D and E are on such courses - the same figure as in 1996.
Since then, the National Institute of Adult and Continuing Education has been counting the proportion of adults participating in learning through an annual survey.
Its director, Professor Alan Tuckett, said this year's survey findings posed sharp challenges for a government trying to improve the nation's skills.
The government is committed to a major programme of increasing the skills of working adults and youngsters, so that Britain can compete with the fast growing economies of China and elsewhere.
It has re-focused much of its funding away from short courses, in areas like foreign languages, towards improving basic skills such as literacy and numeracy.
And it has mounted several high-profile TV advertising campaigns to try to boost retraining. The government plans to spend £20m on advertising adult education before 2012.
Professor Tuckett said: "The very groups identified as key to the achievement of the skills strategy and in the Leitch Review are bearing the heaviest burden of the re-balancing of funding.
"The findings suggest that the price of investment in key groups of adults in workplace learning is being paid for by reduced participation by other adults from exactly the same groups.
"This is either because other workplace learning opportunities have declined, or because those adults can no longer access public provision they previously chose for themselves."
He added that there was a great deal of demoralisation among providers of adult education as more and more classes were closing.
The Department for Universities, Innovation and Skills (Dius) said the figures hid a more complex picture.
Skills minister David Lammy said the government was committed to ensuring those in the greatest need could access the skills training they needed to help them get a job or advance at work.
He said the government had invested £3bn in the last seven years and had helped more than 1.75 million adults improve their literacy and numeracy skills - though these government statistics count "adults" as being those over 16.
Mr Lammy added that funding for English as a Second Language courses had trebled since 2001, and that the budget for work-based learning was being raised to £1bn a year.
"To improve value for money we have stopped funding many short courses and are focusing this money towards, higher quality courses that directly improve people's employability," he added.
It is from these courses that many of the learners had been lost, Dius said.