The body says there were impressive gains earlier on
The proportion of adult learners in the UK is at its lowest level since Labour came to power, a survey suggests.
A study among nearly 5,000 people found those in the highest socio-economic groups were twice as likely to study as those in the poorest.
The National Institute of Adult Continuing Education which ran the survey, says the results are worrying.
The government rejects the findings, saying "results from Niace surveys have been fairly volatile over the years".
According to the study, there is a "widening gap between the educationally privileged and the educationally excluded".
Although there had been a one point rise in the proportion of adults who are or have been studying in the past three years, there had been a two point fall in those currently studying (down from 20% in 2008 to 18% this year). In 1996, the level was 23%.
The survey suggests the level of adult learning among the lowest socio-economic groups had "fallen to a 10-year-low", with 24% of that group either studying now or in the past three years.
This compares with a rate of 53% among the highest group.
While just 20% of this group said they had done no learning since leaving school, the figure rose to 55% among the least well-off.
Alan Tuckett, chief executive of Niace said: "These findings are sobering for a government that has invested 52% more in real terms in post-compulsory education and training since its 1997 election.
"After impressive gains in its first five years in office there has been a marked decline in participation since the adoption of its skills strategies in 2003, 2005 and 2006."
"Overall, the 2009 Niace survey suggests that the time has come for government to consider the price paid, in England at least, for its skills strategy.
"It is clear that the opportunity to gain a first qualification for a small cohort of the least qualified is bought at the expense of engagement by large numbers of others from the same groups."
A spokeswoman for England's Department for Innovation, Universities and Skills said: "The results from Niace surveys have been fairly volatile over the years, and not too much reliance should be put on a single year's results.
"Our National Adult Learner Survey, last conducted in 2005, showed a substantially higher level of people participating in learning, and an increase in participation amongst those from lower socio-economic groups."
She added that the government would soon undertake another survey of adult learners and was looking at establishing an indicator to "capture the contribution learning makes to well-being and social inclusion".