The government has campaigned to raise adult literacy levels
Adult education courses are attracting fewer people, despite government initiatives to improve basic skills.
The Conservatives say that the number of people enrolling on adult education courses has fallen compared to when they were in government.
The number of enrolments has gone down by 190,000 since 1996, say the Conservatives - a figure that education spokesperson, Damian Green, describes as "shocking and surprising".
The Conservatives say that this is another example of government initiatives in education failing to deliver, despite substantial spending and publicity.
And they say that the government's ambitions to raise the basic skills levels of adults will be "undermined" by the lack of take-up for courses.
"These are genuinely both shocking and surprising figures given the amount of ministerial rhetoric that we hear about re-skilling Britain," said Mr Green.
"To discover that the government has spent £663m on adult education, and yet the number of enrolments are tens of thousand less than when they took office, is another sign of how the initiative culture is failing the most vulnerable in our society."
In response, the Department for Education and Skills said that the national Adult Learning Survey, published in April 2003, indicated that 76% of adults aged 16 to 69 (or 31million) have engaged in learning over the last three years.
"Supporting and encouraging adults to participate in learning are key parts of the government's agenda.
"That is why we will be publishing a Skills Strategy shortly which focuses on the importance of adult skills," said an education department spokesperson.